[v6,3/6] drm/dp_mst: Start tracking per-port VCPI allocations

Commit Message

Lyude Paul Nov. 16, 2018, 12:50 a.m. UTC

There has been a TODO waiting for quite a long time in
drm_dp_mst_topology.c:

	/* We cannot rely on port->vcpi.num_slots to update
	 * topology_state->avail_slots as the port may not exist if the parent
	 * branch device was unplugged. This should be fixed by tracking
	 * per-port slot allocation in drm_dp_mst_topology_state instead of
	 * depending on the caller to tell us how many slots to release.
	 */

That's not the only reason we should fix this: forcing the driver to
track the VCPI allocations throughout a state's atomic check is
error prone, because it means that extra care has to be taken with the
order that drm_dp_atomic_find_vcpi_slots() and
drm_dp_atomic_release_vcpi_slots() are called in in order to ensure
idempotency. Currently the only driver actually using these helpers,
i915, doesn't even do this correctly: multiple ->best_encoder() checks
with i915's current implementation would not be idempotent and would
over-allocate VCPI slots, something I learned trying to implement
fallback retraining in MST.

So: simplify this whole mess, and teach drm_dp_atomic_find_vcpi_slots()
and drm_dp_atomic_release_vcpi_slots() to track the VCPI allocations for
each port. This allows us to ensure idempotency without having to rely
on the driver as much. Additionally: the driver doesn't need to do any
kind of VCPI slot tracking anymore if it doesn't need it for it's own
internal state.

Additionally; this adds a new drm_dp_mst_atomic_check() helper which
must be used by atomic drivers to perform validity checks for the new
VCPI allocations incurred by a state.

Also: update the documentation and make it more obvious that these
/must/ be called by /all/ atomic drivers supporting MST.

Changes since v6:
 - Keep a kref to all of the ports we have allocations on. This required
   a good bit of changing to when we call drm_dp_find_vcpi_slots(),
   mainly that we need to ensure that we only redo VCPI allocations on
   actual mode or CRTC changes, not crtc_state->active changes.
   Additionally, we no longer take the registration of the DRM connector
   for each port into account because so long as we have a kref to the
   port in the new or previous atomic state, the connector will stay
   registered.
 - Use the small changes to drm_dp_put_port() to add even more error
   checking to make misusage of the helpers more obvious. I added this
   after having to chase down various use-after-free conditions that
   started popping up from the new helpers so no one else has to
   troubleshoot that.
 - Move some accidental DRM_DEBUG_KMS() calls to DRM_DEBUG_ATOMIC()
 - Update documentation again, note that find/release() should both not be
   called on the same port in a single atomic check phase (but multiple
   calls to one or the other is OK)

Changes since v4:
 - Don't skip the atomic checks for VCPI allocations if no new VCPI
   allocations happen in a state. This makes the next change I'm about
   to list here a lot easier to implement.
 - Don't ignore VCPI allocations on destroyed ports, instead ensure that
   when ports are destroyed and still have VCPI allocations in the
   topology state, the only state changes allowed are releasing said
   ports' VCPI. This prevents a state with a mix of VCPI allocations
   from destroyed ports, and allocations from valid ports.

Changes since v3:
 - Don't release VCPI allocations in the topology state immediately in
   drm_dp_atomic_release_vcpi_slots(), instead mark them as 0 and skip
   over them in drm_dp_mst_duplicate_state(). This makes it so
   drm_dp_atomic_release_vcpi_slots() is still idempotent while also
   throwing warnings if the driver messes up it's book keeping and tries
   to release VCPI slots on a port that doesn't have any pre-existing
   VCPI allocation - danvet
 - Change mst_state/state in some debugging messages to "mst state"

Changes since v2:
 - Use kmemdup() for duplicating MST state - danvet
 - Move port validation out of duplicate state callback - danvet
 - Handle looping through MST topology states in
   drm_dp_mst_atomic_check() so the driver doesn't have to do it
 - Fix documentation in drm_dp_atomic_find_vcpi_slots()
 - Move the atomic check for each individual topology state into it's
   own function, reduces indenting
 - Don't consider "stale" MST ports when calculating the bandwidth
   requirements. This is needed because originally we relied on the
   state duplication functions to prune any stale ports from the new
   state, which would prevent us from incorrectly considering their
   bandwidth requirements alongside legitimate new payloads.
 - Add function references in drm_dp_atomic_release_vcpi_slots() - danvet
 - Annotate atomic VCPI and atomic check functions with __must_check
   - danvet

Changes since v1:
 - Don't use the now-removed ->atomic_check() for private objects hook,
   just give drivers a function to call themselves

Signed-off-by: Lyude Paul <[e-mail]>
Cc: Daniel Vetter <[e-mail]>
---
 drivers/gpu/drm/drm_dp_mst_topology.c | 294 +++++++++++++++++++++++---
 drivers/gpu/drm/i915/intel_display.c  |   4 +
 drivers/gpu/drm/i915/intel_dp_mst.c   |  64 +++---
 include/drm/drm_dp_mst_helper.h       |  24 ++-
 4 files changed, 319 insertions(+), 67 deletions(-)

Comments

Daniel Vetter Nov. 26, 2018, 9:04 p.m. UTC | #1

On Thu, Nov 15, 2018 at 07:50:05PM -0500, Lyude Paul wrote:
> There has been a TODO waiting for quite a long time in
> drm_dp_mst_topology.c:
> 
> 	/* We cannot rely on port->vcpi.num_slots to update
> 	 * topology_state->avail_slots as the port may not exist if the parent
> 	 * branch device was unplugged. This should be fixed by tracking
> 	 * per-port slot allocation in drm_dp_mst_topology_state instead of
> 	 * depending on the caller to tell us how many slots to release.
> 	 */
> 
> That's not the only reason we should fix this: forcing the driver to
> track the VCPI allocations throughout a state's atomic check is
> error prone, because it means that extra care has to be taken with the
> order that drm_dp_atomic_find_vcpi_slots() and
> drm_dp_atomic_release_vcpi_slots() are called in in order to ensure
> idempotency. Currently the only driver actually using these helpers,
> i915, doesn't even do this correctly: multiple ->best_encoder() checks
> with i915's current implementation would not be idempotent and would
> over-allocate VCPI slots, something I learned trying to implement
> fallback retraining in MST.
> 
> So: simplify this whole mess, and teach drm_dp_atomic_find_vcpi_slots()
> and drm_dp_atomic_release_vcpi_slots() to track the VCPI allocations for
> each port. This allows us to ensure idempotency without having to rely
> on the driver as much. Additionally: the driver doesn't need to do any
> kind of VCPI slot tracking anymore if it doesn't need it for it's own
> internal state.
> 
> Additionally; this adds a new drm_dp_mst_atomic_check() helper which
> must be used by atomic drivers to perform validity checks for the new
> VCPI allocations incurred by a state.
> 
> Also: update the documentation and make it more obvious that these
> /must/ be called by /all/ atomic drivers supporting MST.
> 
> Changes since v6:
>  - Keep a kref to all of the ports we have allocations on. This required
>    a good bit of changing to when we call drm_dp_find_vcpi_slots(),
>    mainly that we need to ensure that we only redo VCPI allocations on
>    actual mode or CRTC changes, not crtc_state->active changes.
>    Additionally, we no longer take the registration of the DRM connector
>    for each port into account because so long as we have a kref to the
>    port in the new or previous atomic state, the connector will stay
>    registered.

I write an entire pile of small nitpits (still included most of them
below), until I realized this here wont work. Delaying the call to destroy
the connector (well, unregister it really) wreaks the design we've come up
with thus far, resulting in most of my comments here.

Instead, all we need to do is delay the kfree(port) at the bottom of
drm_dp_destroy_port(). The vcpi allocation structure _only_ needs the
pointer value to stay valid, as a lookup key. It doesn't care at all about
anything actually stored in there. So the only thing we need to delay is
the kfree. I think the simplest way to achieve this is to add a 2nd kref
(port->kfree_ref or something like that), with on reference held by the
port itself (released in drm_dp_destroy_port), and the other one held
as-needed by the vcpi allocation lists.

I think if we go with this design instead of retrofitting a semantic
change of the port lifetime itself, all the complications I complain about
below should disappear.

Piles of comments below.

Cheers, Daniel

>  - Use the small changes to drm_dp_put_port() to add even more error
>    checking to make misusage of the helpers more obvious. I added this
>    after having to chase down various use-after-free conditions that
>    started popping up from the new helpers so no one else has to
>    troubleshoot that.
>  - Move some accidental DRM_DEBUG_KMS() calls to DRM_DEBUG_ATOMIC()
>  - Update documentation again, note that find/release() should both not be
>    called on the same port in a single atomic check phase (but multiple
>    calls to one or the other is OK)
> 
> Changes since v4:
>  - Don't skip the atomic checks for VCPI allocations if no new VCPI
>    allocations happen in a state. This makes the next change I'm about
>    to list here a lot easier to implement.
>  - Don't ignore VCPI allocations on destroyed ports, instead ensure that
>    when ports are destroyed and still have VCPI allocations in the
>    topology state, the only state changes allowed are releasing said
>    ports' VCPI. This prevents a state with a mix of VCPI allocations
>    from destroyed ports, and allocations from valid ports.
> 
> Changes since v3:
>  - Don't release VCPI allocations in the topology state immediately in
>    drm_dp_atomic_release_vcpi_slots(), instead mark them as 0 and skip
>    over them in drm_dp_mst_duplicate_state(). This makes it so
>    drm_dp_atomic_release_vcpi_slots() is still idempotent while also
>    throwing warnings if the driver messes up it's book keeping and tries
>    to release VCPI slots on a port that doesn't have any pre-existing
>    VCPI allocation - danvet
>  - Change mst_state/state in some debugging messages to "mst state"
> 
> Changes since v2:
>  - Use kmemdup() for duplicating MST state - danvet
>  - Move port validation out of duplicate state callback - danvet
>  - Handle looping through MST topology states in
>    drm_dp_mst_atomic_check() so the driver doesn't have to do it
>  - Fix documentation in drm_dp_atomic_find_vcpi_slots()
>  - Move the atomic check for each individual topology state into it's
>    own function, reduces indenting
>  - Don't consider "stale" MST ports when calculating the bandwidth
>    requirements. This is needed because originally we relied on the
>    state duplication functions to prune any stale ports from the new
>    state, which would prevent us from incorrectly considering their
>    bandwidth requirements alongside legitimate new payloads.
>  - Add function references in drm_dp_atomic_release_vcpi_slots() - danvet
>  - Annotate atomic VCPI and atomic check functions with __must_check
>    - danvet
> 
> Changes since v1:
>  - Don't use the now-removed ->atomic_check() for private objects hook,
>    just give drivers a function to call themselves
> 
> Signed-off-by: Lyude Paul <[e-mail]>
> Cc: Daniel Vetter <[e-mail]>
> ---
>  drivers/gpu/drm/drm_dp_mst_topology.c | 294 +++++++++++++++++++++++---
>  drivers/gpu/drm/i915/intel_display.c  |   4 +
>  drivers/gpu/drm/i915/intel_dp_mst.c   |  64 +++---
>  include/drm/drm_dp_mst_helper.h       |  24 ++-
>  4 files changed, 319 insertions(+), 67 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
> index 00fbe7a2699d..183d0e832ced 100644
> --- a/drivers/gpu/drm/drm_dp_mst_topology.c
> +++ b/drivers/gpu/drm/drm_dp_mst_topology.c
> @@ -2617,21 +2617,42 @@ static int drm_dp_init_vcpi(struct drm_dp_mst_topology_mgr *mgr,
>  }
>  
>  /**
> - * drm_dp_atomic_find_vcpi_slots() - Find and add vcpi slots to the state
> + * drm_dp_atomic_find_vcpi_slots() - Find and add VCPI slots to the state
>   * @state: global atomic state
>   * @mgr: MST topology manager for the port
>   * @port: port to find vcpi slots for
>   * @pbn: bandwidth required for the mode in PBN
>   *
> - * RETURNS:
> - * Total slots in the atomic state assigned for this port or error
> + * Allocates VCPI slots to @port, replacing any previous VCPI allocations it
> + * may have had. Any atomic drivers which support MST must call this function
> + * in their &drm_encoder_helper_funcs.atomic_check() callback to change the
> + * current VCPI allocation for the new state, but only when
> + * &drm_crtc_state.mode_changed or &drm_crtc_state.connectors_changed is set
> + * to ensure compatibility with userspace applications that still use the
> + * legacy modesetting UAPI.
> + *
> + * Allocations set by this function are not checked against the bandwidth
> + * restraints of @mgr until the driver calls drm_dp_mst_atomic_check().
> + *
> + * Additionally, it is OK to call this function multiple times on the same
> + * @port as needed. It is not OK however, to call this function and
> + * drm_dp_atomic_release_vcpi_slots() in the same atomic check phase.
> + *
> + * See also:
> + * drm_dp_atomic_release_vcpi_slots()
> + * drm_dp_mst_atomic_check()
> + *
> + * Returns:
> + * Total slots in the atomic state assigned for this port, or a negative error
> + * code if the port no longer exists
>   */
>  int drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state *state,
>  				  struct drm_dp_mst_topology_mgr *mgr,
>  				  struct drm_dp_mst_port *port, int pbn)
>  {
>  	struct drm_dp_mst_topology_state *topology_state;
> -	int req_slots;
> +	struct drm_dp_vcpi_allocation *pos, *vcpi = NULL;
> +	int prev_slots, req_slots, ret;
>  
>  	topology_state = drm_atomic_get_mst_topology_state(state, mgr);
>  	if (IS_ERR(topology_state))
> @@ -2640,20 +2661,60 @@ int drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state *state,
>  	port = drm_dp_get_validated_port_ref(mgr, port);
>  	if (port == NULL)
>  		return -EINVAL;
> -	req_slots = DIV_ROUND_UP(pbn, mgr->pbn_div);
> -	DRM_DEBUG_KMS("vcpi slots req=%d, avail=%d\n",
> -			req_slots, topology_state->avail_slots);
>  
> -	if (req_slots > topology_state->avail_slots) {
> -		drm_dp_put_port(port);
> -		return -ENOSPC;
> +	topology_state->vcpi_allocated = true;
> +
> +	/* Find the current allocation for this port, if any */
> +	list_for_each_entry(pos, &topology_state->vcpis, next) {
> +		if (pos->port == port) {
> +			vcpi = pos;
> +			prev_slots = vcpi->vcpi;
> +
> +			/*
> +			 * This should never happen, unless the driver tries
> +			 * releasing and allocating the same VCPI allocation,
> +			 * which is an error
> +			 */
> +			if (WARN_ON(!prev_slots)) {


> +				DRM_ERROR("cannot allocate and release VCPI on [MST PORT:%p] in the same state\n",
> +					  port);
> +				return -EINVAL;
> +			}

Allowing atomic changes to output state (like disabling it on one crtc,
and then enabling on the other, thus both releasing and re-allocating the
vcpi in one go) is kinda the point of atomic. So definitely can't EINVAL
here.

Sounds like we need more testcases in igt?

> +
> +			break;
> +		}
>  	}
> +	if (!vcpi)
> +		prev_slots = 0;
>  
> -	topology_state->avail_slots -= req_slots;
> -	DRM_DEBUG_KMS("vcpi slots avail=%d", topology_state->avail_slots);
> +	req_slots = DIV_ROUND_UP(pbn, mgr->pbn_div);
> +
> +	DRM_DEBUG_ATOMIC("[CONNECTOR:%d:%s] [MST PORT:%p] VCPI %d -> %d\n",
> +			 port->connector->base.id, port->connector->name,
> +			 port, prev_slots, req_slots);
> +
> +	/* Add the new allocation to the state */
> +	if (!vcpi) {
> +		vcpi = kzalloc(sizeof(*vcpi), GFP_KERNEL);
> +		if (!vcpi) {
> +			ret = -ENOMEM;
> +			goto out;
> +		}
>  
> +		vcpi->port = drm_dp_get_validated_port_ref(mgr, port);
> +		if (!vcpi->port) {
> +			ret = -EINVAL;
> +			kfree(vcpi);
> +			goto out;
> +		}
> +		list_add(&vcpi->next, &topology_state->vcpis);
> +	}
> +	vcpi->vcpi = req_slots;
> +
> +	ret = req_slots;
> +out:
>  	drm_dp_put_port(port);
> -	return req_slots;
> +	return ret;
>  }
>  EXPORT_SYMBOL(drm_dp_atomic_find_vcpi_slots);
>  
> @@ -2661,31 +2722,66 @@ EXPORT_SYMBOL(drm_dp_atomic_find_vcpi_slots);
>   * drm_dp_atomic_release_vcpi_slots() - Release allocated vcpi slots
>   * @state: global atomic state
>   * @mgr: MST topology manager for the port
> - * @slots: number of vcpi slots to release
> + * @port: The port to release the VCPI slots from
>   *
> - * RETURNS:
> - * 0 if @slots were added back to &drm_dp_mst_topology_state->avail_slots or
> - * negative error code
> + * Releases any VCPI slots that have been allocated to a port in the atomic
> + * state. Any atomic drivers which support MST must call this function in
> + * their &drm_connector_helper_funcs.atomic_check() callback when the
> + * connector will no longer have VCPI allocated (e.g. because it's CRTC was
> + * removed) when it had VCPI allocated in the previous atomic state.
> + *
> + * It is OK to call this even if @port has been removed from the system.
> + * Additionally, it is OK to call this function multiple times on the same
> + * @port as needed. It is not OK however, to call this function and
> + * drm_dp_atomic_find_vcpi_slots() on the same @port in a single atomic check
> + * phase.
> + *
> + * See also:
> + * drm_dp_atomic_find_vcpi_slots()
> + * drm_dp_mst_atomic_check()
> + *
> + * Returns:
> + * 0 if all slots for this port were added back to
> + * &drm_dp_mst_topology_state.avail_slots or negative error code
>   */
>  int drm_dp_atomic_release_vcpi_slots(struct drm_atomic_state *state,
>  				     struct drm_dp_mst_topology_mgr *mgr,
> -				     int slots)
> +				     struct drm_dp_mst_port *port)
>  {
>  	struct drm_dp_mst_topology_state *topology_state;
> +	struct drm_dp_vcpi_allocation *pos;
> +	bool found = false;
>  
>  	topology_state = drm_atomic_get_mst_topology_state(state, mgr);
>  	if (IS_ERR(topology_state))
>  		return PTR_ERR(topology_state);
>  
> -	/* We cannot rely on port->vcpi.num_slots to update
> -	 * topology_state->avail_slots as the port may not exist if the parent
> -	 * branch device was unplugged. This should be fixed by tracking
> -	 * per-port slot allocation in drm_dp_mst_topology_state instead of
> -	 * depending on the caller to tell us how many slots to release.
> -	 */
> -	topology_state->avail_slots += slots;
> -	DRM_DEBUG_KMS("vcpi slots released=%d, avail=%d\n",
> -			slots, topology_state->avail_slots);
> +	list_for_each_entry(pos, &topology_state->vcpis, next) {

> +		if (pos->port == port) {
> +			found = true;
> +			break;
> +		}
> +	}
> +	if (WARN_ON(!found)) {
> +		DRM_ERROR("no VCPI for [MST PORT:%p] found in mst state %p\n",
> +			  port, &topology_state->base);
> +		return -EINVAL;
> +	}
> +
> +	DRM_DEBUG_ATOMIC("[MST PORT:%p] VCPI %d -> 0\n", port, pos->vcpi);
> +	if (pos->vcpi) {
> +		/*
> +		 * This should be safe-as the previous state will still hold a
> +		 * reference to this port until we swap states. If it's not,
> +		 * the driver has a bookkeeping error
> +		 */
> +		if (WARN_ON(drm_dp_put_port(port))) {

The usual way to do this (instead of adding a return value to the normal
_put function) is something like

	kref_put(port, function_that_with_just_a_WARNING_DRM_ERROR)

avoids the need for patch 2.

> +			DRM_ERROR("cannot allocate and release VCPI on [MST PORT:%p] in the same state\n",
> +				  port);
> +			return -EINVAL;
> +		}
> +		pos->vcpi = 0;
> +	}
>  
>  	return 0;
>  }
> @@ -3115,15 +3211,45 @@ static void drm_dp_destroy_connector_work(struct work_struct *work)
>  static struct drm_private_state *
>  drm_dp_mst_duplicate_state(struct drm_private_obj *obj)
>  {
> -	struct drm_dp_mst_topology_state *state;
> +	struct drm_dp_mst_topology_state *state, *old_state =
> +		to_dp_mst_topology_state(obj->state);
> +	struct drm_dp_vcpi_allocation *pos, *vcpi;
>  
> -	state = kmemdup(obj->state, sizeof(*state), GFP_KERNEL);
> +	state = kmemdup(old_state, sizeof(*state), GFP_KERNEL);
>  	if (!state)
>  		return NULL;
>  
>  	__drm_atomic_helper_private_obj_duplicate_state(obj, &state->base);
>  
> +	INIT_LIST_HEAD(&state->vcpis);
> +	state->vcpi_allocated = false;
> +
> +	list_for_each_entry(pos, &old_state->vcpis, next) {
> +		/* Prune leftover freed VCPI allocations */
> +		if (!pos->vcpi)
> +			continue;
> +
> +		vcpi = kmemdup(pos, sizeof(*vcpi), GFP_KERNEL);
> +		if (!vcpi)
> +			goto fail;
> +
> +		/* Take a plain old kref here and don't validate the port, as
> +		 * it's topology might not even exist anymore
> +		 */
> +		kref_get(&vcpi->port->kref);
> +		list_add(&vcpi->next, &state->vcpis);
> +	}
> +
>  	return &state->base;
> +
> +fail:
> +	list_for_each_entry_safe(pos, vcpi, &state->vcpis, next) {
> +		drm_dp_put_port(pos->port);
> +		kfree(pos);
> +	}
> +	kfree(state);
> +
> +	return NULL;
>  }
>  
>  static void drm_dp_mst_destroy_state(struct drm_private_obj *obj,
> @@ -3131,14 +3257,116 @@ static void drm_dp_mst_destroy_state(struct drm_private_obj *obj,
>  {
>  	struct drm_dp_mst_topology_state *mst_state =
>  		to_dp_mst_topology_state(state);
> +	struct drm_dp_vcpi_allocation *pos, *tmp;
> +
> +	list_for_each_entry_safe(pos, tmp, &mst_state->vcpis, next) {
> +		/* We only keep references to ports with non-zero VCPIs */
> +		if (pos->vcpi)
> +			drm_dp_put_port(pos->port);
> +		kfree(pos);
> +	}
>  
>  	kfree(mst_state);
>  }
>  
> -static const struct drm_private_state_funcs mst_state_funcs = {
> +static inline int
> +drm_dp_mst_atomic_check_topology_state(struct drm_dp_mst_topology_mgr *mgr,
> +				       struct drm_dp_mst_topology_state *mst_state)
> +{
> +	struct drm_dp_vcpi_allocation *vcpi;
> +	struct drm_dp_mst_port *port;
> +	int avail_slots = 63, ret;
> +
> +	/* There's no possible scenario where releasing VCPI or keeping it the
> +	 * same would make the state invalid
> +	 */
> +	if (!mst_state->vcpi_allocated) {

Do we need this? If not I'm always in favour of removing code if it's not
needed :-)

> +		DRM_DEBUG_ATOMIC("[MST MGR:%p] state %p allocates no VCPI, check skipped\n",
> +				 mgr, &mst_state->base);
> +		return 0;
> +	}
> +
> +	list_for_each_entry(vcpi, &mst_state->vcpis, next) {
> +		/* Releasing VCPI is always OK-even if the port is gone */
> +		if (!vcpi->vcpi) {
> +			DRM_DEBUG_ATOMIC("[MST PORT:%p] releases all VCPI slots\n",
> +					 vcpi->port);
> +			continue;
> +		}
> +
> +		port = drm_dp_get_validated_port_ref(mgr, vcpi->port);
> +		if (!port) {
> +			DRM_DEBUG_ATOMIC("[MST PORT:%p] is gone but still has VCPI, cannot add new VCPI\n",
> +					 vcpi->port);
> +			return -EINVAL;
> +		}

For find_vcpi we've already done this. For unplugged vcpi, we can't do
this here I think - otherwise if you enable 2 mst outputs on the same
branch, and unplug both, old non-atomic userspace can never ever again
disable them because it can't disable both. The check (to skip already
deallocated ports) above isn't enough to handle that.

> +
> +		DRM_DEBUG_ATOMIC("[MST PORT:%p] requires %d vcpi slots\n",
> +				 vcpi->port, vcpi->vcpi);
> +
> +		avail_slots -= vcpi->vcpi;
> +		if (avail_slots < 0) {
> +			DRM_DEBUG_ATOMIC("[MST PORT:%p] not enough VCPI slots in mst state %p (avail=%d)\n",
> +					 vcpi->port, mst_state,
> +					 avail_slots + vcpi->vcpi);
> +			ret = -ENOSPC;
> +			goto port_fail;
> +		}
> +
> +		drm_dp_put_port(port);
> +	}
> +	DRM_DEBUG_ATOMIC("[MST MGR:%p] mst state %p VCPI avail=%d used=%d\n",
> +			 mgr, mst_state, avail_slots,
> +			 63 - avail_slots);
> +
> +	return 0;
> +port_fail:
> +	drm_dp_put_port(port);
> +	return ret;
> +}
> +
> +/**
> + * drm_dp_mst_atomic_check - Check that the new state of an MST topology in an
> + * atomic update is valid
> + * @state: Pointer to the new &struct drm_dp_mst_topology_state
> + *
> + * Checks the given topology state for an atomic update to ensure that it's
> + * valid. This includes checking whether there's enough bandwidth to support
> + * the new VCPI allocations in the atomic update.
> + *
> + * Any atomic drivers supporting DP MST must make sure to call this after
> + * checking the rest of their state in their
> + * &drm_mode_config_funcs.atomic_check() callback.
> + *
> + * See also:
> + * drm_dp_atomic_find_vcpi_slots()
> + * drm_dp_atomic_release_vcpi_slots()
> + *
> + * Returns:
> + *
> + * 0 if the new state is valid, negative error code otherwise.
> + */
> +int drm_dp_mst_atomic_check(struct drm_atomic_state *state)
> +{
> +	struct drm_dp_mst_topology_mgr *mgr;
> +	struct drm_dp_mst_topology_state *mst_state;
> +	int i, ret = 0;
> +
> +	for_each_new_mst_mgr_in_state(state, mgr, mst_state, i) {
> +		ret = drm_dp_mst_atomic_check_topology_state(mgr, mst_state);
> +		if (ret)
> +			break;
> +	}
> +
> +	return ret;
> +}
> +EXPORT_SYMBOL(drm_dp_mst_atomic_check);
> +
> +const struct drm_private_state_funcs drm_dp_mst_topology_state_funcs = {
>  	.atomic_duplicate_state = drm_dp_mst_duplicate_state,
>  	.atomic_destroy_state = drm_dp_mst_destroy_state,
>  };
> +EXPORT_SYMBOL(drm_dp_mst_topology_state_funcs);
>  
>  /**
>   * drm_atomic_get_mst_topology_state: get MST topology state
> @@ -3216,13 +3444,11 @@ int drm_dp_mst_topology_mgr_init(struct drm_dp_mst_topology_mgr *mgr,
>  		return -ENOMEM;
>  
>  	mst_state->mgr = mgr;
> -
> -	/* max. time slots - one slot for MTP header */
> -	mst_state->avail_slots = 63;
> +	INIT_LIST_HEAD(&mst_state->vcpis);
>  
>  	drm_atomic_private_obj_init(&mgr->base,
>  				    &mst_state->base,
> -				    &mst_state_funcs);
> +				    &drm_dp_mst_topology_state_funcs);
>  
>  	return 0;
>  }
> diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
> index 132e978227fb..8e85e04a5948 100644
> --- a/drivers/gpu/drm/i915/intel_display.c
> +++ b/drivers/gpu/drm/i915/intel_display.c
> @@ -12592,6 +12592,10 @@ static int intel_atomic_check(struct drm_device *dev,
>  				       "[modeset]" : "[fastset]");
>  	}
>  
> +	ret = drm_dp_mst_atomic_check(state);
> +	if (ret)
> +		return ret;
> +
>  	if (any_ms) {
>  		ret = intel_modeset_checks(state);
>  
> diff --git a/drivers/gpu/drm/i915/intel_dp_mst.c b/drivers/gpu/drm/i915/intel_dp_mst.c
> index 4de247ddf05f..be2d1ac7d87d 100644
> --- a/drivers/gpu/drm/i915/intel_dp_mst.c
> +++ b/drivers/gpu/drm/i915/intel_dp_mst.c
> @@ -41,8 +41,13 @@ static bool intel_dp_mst_compute_config(struct intel_encoder *encoder,
>  	struct drm_connector *connector = conn_state->connector;
>  	void *port = to_intel_connector(connector)->port;
>  	struct drm_atomic_state *state = pipe_config->base.state;
> +	struct drm_crtc *crtc = pipe_config->base.crtc;
> +	struct drm_crtc_state *old_crtc_state =
> +		drm_atomic_get_old_crtc_state(state, crtc);
> +	struct drm_crtc_state *new_crtc_state = &pipe_config->base;
>  	int bpp;
> -	int lane_count, slots = 0;
> +	int lane_count, slots =
> +		to_intel_crtc_state(old_crtc_state)->dp_m_n.tu;
>  	const struct drm_display_mode *adjusted_mode = &pipe_config->base.adjusted_mode;
>  	int mst_pbn;
>  	bool constant_n = drm_dp_has_quirk(&intel_dp->desc,
> @@ -77,8 +82,12 @@ static bool intel_dp_mst_compute_config(struct intel_encoder *encoder,
>  	mst_pbn = drm_dp_calc_pbn_mode(adjusted_mode->crtc_clock, bpp);
>  	pipe_config->pbn = mst_pbn;
>  
> -	/* Zombie connectors can't have VCPI slots */
> -	if (!drm_connector_is_unregistered(connector)) {
> +	/* Only change VCPI allocation on actual mode changes, to prevent us
> +	 * from trying to allocate VCPI to ports that no longer exist when we
> +	 * may just be trying to disable DPMS on them
> +	 */
> +	if (new_crtc_state->mode_changed ||
> +	    new_crtc_state->connectors_changed) {

Why exactly is this needed? We only call compute_config on the enocoder
when enabling an output. Not when disabling one. Similar for the atomic
helpers and the various encoder callbacks (you have the same change in the
nouveau code). Did this actually blow up while testing?

>  		slots = drm_dp_atomic_find_vcpi_slots(state,
>  						      &intel_dp->mst_mgr,
>  						      port,
> @@ -107,36 +116,39 @@ static bool intel_dp_mst_compute_config(struct intel_encoder *encoder,
>  	return true;
>  }
>  
> -static int intel_dp_mst_atomic_check(struct drm_connector *connector,
> -		struct drm_connector_state *new_conn_state)
> +static int
> +intel_dp_mst_atomic_check(struct drm_connector *connector,
> +			  struct drm_connector_state *new_conn_state)
>  {
> +	struct intel_connector *intel_connector =
> +		to_intel_connector(connector);
> +	struct drm_dp_mst_topology_mgr *mgr =
> +		&intel_connector->mst_port->mst_mgr;
> +	struct drm_dp_mst_port *port = intel_connector->port;
>  	struct drm_atomic_state *state = new_conn_state->state;
> -	struct drm_connector_state *old_conn_state;
> -	struct drm_crtc *old_crtc;
> -	struct drm_crtc_state *crtc_state;
> -	int slots, ret = 0;
> +	struct drm_connector_state *old_conn_state =
> +		drm_atomic_get_old_connector_state(state, connector);
> +	struct drm_crtc *old_crtc = old_conn_state->crtc,
> +			*new_crtc = new_conn_state->crtc;
> +	struct drm_crtc_state *old_crtc_state, *new_crtc_state;
>  
> -	old_conn_state = drm_atomic_get_old_connector_state(state, connector);
> -	old_crtc = old_conn_state->crtc;

Bikeshed: Reworking all the above hides the real changes quite a bit.

>  	if (!old_crtc)
> -		return ret;
> +		return 0;
>  
> -	crtc_state = drm_atomic_get_new_crtc_state(state, old_crtc);
> -	slots = to_intel_crtc_state(crtc_state)->dp_m_n.tu;
> -	if (drm_atomic_crtc_needs_modeset(crtc_state) && slots > 0) {
> -		struct drm_dp_mst_topology_mgr *mgr;
> -		struct drm_encoder *old_encoder;
> +	old_crtc_state = drm_atomic_get_old_crtc_state(state, old_crtc);
> +	if (!old_crtc_state ||

!old_crtc_state would be a framework bug. Just remove that check.

> +	    !to_intel_crtc_state(old_crtc_state)->dp_m_n.tu)
> +		return 0;

Hm, why do we need this? Having a DP port previously enabled with a 0 tu
sounds like a pretty big driver bug.
>  
> -		old_encoder = old_conn_state->best_encoder;
> -		mgr = &enc_to_mst(old_encoder)->primary->dp.mst_mgr;
> +	/* If we switched CRTCs, clear the previous one's allocation */
> +	new_crtc_state = drm_atomic_get_new_crtc_state(state, old_crtc);
> +	if (new_crtc_state->connectors_changed && !new_crtc_state->enable)
> +		to_intel_crtc_state(new_crtc_state)->dp_m_n.tu = 0;

No idea why we need to clear this here? Shouldn't we just clear this
unconditionally if we release the allocation.

Also naming convention, I'd call this old_crtc_new_state, and maybe
old_crtc_old_state. To make it clear these are the old/new states for
old_crtc. Instead of old/new state for the new crtc.

>  
> -		ret = drm_dp_atomic_release_vcpi_slots(state, mgr, slots);
> -		if (ret)
> -			DRM_DEBUG_KMS("failed releasing %d vcpi slots:%d\n", slots, ret);
> -		else
> -			to_intel_crtc_state(crtc_state)->dp_m_n.tu = 0;
> -	}
> -	return ret;
> +	if (new_crtc)
> +		return 0;

Not sure why we shouldn't release this? If we handle this in helpers, it
would remove piles of special casing code here I think. Plus remove
special code in the hlpers.


> +
> +	return drm_dp_atomic_release_vcpi_slots(state, mgr, port);
>  }
>  
>  static void intel_mst_disable_dp(struct intel_encoder *encoder,
> diff --git a/include/drm/drm_dp_mst_helper.h b/include/drm/drm_dp_mst_helper.h
> index 3faceb66f5cb..2392e12b6bce 100644
> --- a/include/drm/drm_dp_mst_helper.h
> +++ b/include/drm/drm_dp_mst_helper.h
> @@ -406,9 +406,16 @@ struct drm_dp_payload {
>  
>  #define to_dp_mst_topology_state(x) container_of(x, struct drm_dp_mst_topology_state, base)
>  
> +struct drm_dp_vcpi_allocation {

I think a comment here that this pointer holds a full reference (even past
when the connector is unplugged) would be good. Otherwise the loop in
release_vcpi would be buggy.

> +	struct drm_dp_mst_port *port;
> +	int vcpi;
> +	struct list_head next;
> +};
> +
>  struct drm_dp_mst_topology_state {
>  	struct drm_private_state base;
> -	int avail_slots;
> +	struct list_head vcpis;
> +	u8 vcpi_allocated;
>  	struct drm_dp_mst_topology_mgr *mgr;
>  };
>  
> @@ -619,14 +626,17 @@ void drm_dp_mst_topology_mgr_suspend(struct drm_dp_mst_topology_mgr *mgr);
>  int drm_dp_mst_topology_mgr_resume(struct drm_dp_mst_topology_mgr *mgr);
>  struct drm_dp_mst_topology_state *drm_atomic_get_mst_topology_state(struct drm_atomic_state *state,
>  								    struct drm_dp_mst_topology_mgr *mgr);
> -int drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state *state,
> -				  struct drm_dp_mst_topology_mgr *mgr,
> -				  struct drm_dp_mst_port *port, int pbn);
> -int drm_dp_atomic_release_vcpi_slots(struct drm_atomic_state *state,
> -				     struct drm_dp_mst_topology_mgr *mgr,
> -				     int slots);
> +int __must_check
> +drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state *state,
> +			      struct drm_dp_mst_topology_mgr *mgr,
> +			      struct drm_dp_mst_port *port, int pbn);
> +int __must_check
> +drm_dp_atomic_release_vcpi_slots(struct drm_atomic_state *state,
> +				 struct drm_dp_mst_topology_mgr *mgr,
> +				 struct drm_dp_mst_port *port);
>  int drm_dp_send_power_updown_phy(struct drm_dp_mst_topology_mgr *mgr,
>  				 struct drm_dp_mst_port *port, bool power_up);
> +int __must_check drm_dp_mst_atomic_check(struct drm_atomic_state *state);
>  
>  extern const struct drm_private_state_funcs drm_dp_mst_topology_state_funcs;
>  
> -- 
> 2.19.1
>

Daniel Vetter Nov. 26, 2018, 9:22 p.m. UTC | #2

On Mon, Nov 26, 2018 at 10:04:21PM +0100, Daniel Vetter wrote:
> On Thu, Nov 15, 2018 at 07:50:05PM -0500, Lyude Paul wrote:
> > There has been a TODO waiting for quite a long time in
> > drm_dp_mst_topology.c:
> > 
> > 	/* We cannot rely on port->vcpi.num_slots to update
> > 	 * topology_state->avail_slots as the port may not exist if the parent
> > 	 * branch device was unplugged. This should be fixed by tracking
> > 	 * per-port slot allocation in drm_dp_mst_topology_state instead of
> > 	 * depending on the caller to tell us how many slots to release.
> > 	 */
> > 
> > That's not the only reason we should fix this: forcing the driver to
> > track the VCPI allocations throughout a state's atomic check is
> > error prone, because it means that extra care has to be taken with the
> > order that drm_dp_atomic_find_vcpi_slots() and
> > drm_dp_atomic_release_vcpi_slots() are called in in order to ensure
> > idempotency. Currently the only driver actually using these helpers,
> > i915, doesn't even do this correctly: multiple ->best_encoder() checks
> > with i915's current implementation would not be idempotent and would
> > over-allocate VCPI slots, something I learned trying to implement
> > fallback retraining in MST.
> > 
> > So: simplify this whole mess, and teach drm_dp_atomic_find_vcpi_slots()
> > and drm_dp_atomic_release_vcpi_slots() to track the VCPI allocations for
> > each port. This allows us to ensure idempotency without having to rely
> > on the driver as much. Additionally: the driver doesn't need to do any
> > kind of VCPI slot tracking anymore if it doesn't need it for it's own
> > internal state.
> > 
> > Additionally; this adds a new drm_dp_mst_atomic_check() helper which
> > must be used by atomic drivers to perform validity checks for the new
> > VCPI allocations incurred by a state.
> > 
> > Also: update the documentation and make it more obvious that these
> > /must/ be called by /all/ atomic drivers supporting MST.
> > 
> > Changes since v6:
> >  - Keep a kref to all of the ports we have allocations on. This required
> >    a good bit of changing to when we call drm_dp_find_vcpi_slots(),
> >    mainly that we need to ensure that we only redo VCPI allocations on
> >    actual mode or CRTC changes, not crtc_state->active changes.
> >    Additionally, we no longer take the registration of the DRM connector
> >    for each port into account because so long as we have a kref to the
> >    port in the new or previous atomic state, the connector will stay
> >    registered.
> 
> I write an entire pile of small nitpits (still included most of them
> below), until I realized this here wont work. Delaying the call to destroy
> the connector (well, unregister it really) wreaks the design we've come up
> with thus far, resulting in most of my comments here.
> 
> Instead, all we need to do is delay the kfree(port) at the bottom of
> drm_dp_destroy_port(). The vcpi allocation structure _only_ needs the
> pointer value to stay valid, as a lookup key. It doesn't care at all about
> anything actually stored in there. So the only thing we need to delay is
> the kfree. I think the simplest way to achieve this is to add a 2nd kref
> (port->kfree_ref or something like that), with on reference held by the
> port itself (released in drm_dp_destroy_port), and the other one held
> as-needed by the vcpi allocation lists.
> 
> I think if we go with this design instead of retrofitting a semantic
> change of the port lifetime itself, all the complications I complain about
> below should disappear.

In the above I meant drm_dp_destroy_port or drm_dp_destroy_connector_work.

Aside: I think creating a kref for the final kfree would also solve a
bunch of other issues in a much neater way: In

commit f038c5b99fc1332f558b495d136d4f433ee65caa
Author: Lyude Paul <[e-mail]>
Date:   Tue Nov 13 17:46:14 2018 -0500

    drm/dp_mst: Skip validating ports during destruction, just ref

we could use that kfree reference to make sure the port pointer is alive.
This of course means that drm_dp_update_payload_part1() would also need to
use the kfree reference for the vcpi allocations. And probably everywhere
else (e.g. in your nouveau series for payload information).

That would give us a very clear separation overall between "port can still
be used because it's not yet unplugged" vs. "port only hangs around
because a bunch of vcpi allocations and other payload things upstream of
the port might still need the port lookup key to free resources". Instead
of again sprinkling complicated conditions and magic tricks all over the
code to figure out whethe we should allow the get_validate_port to succeed
or not, or the modeset to succeed or not.
-Daniel

> 
> Piles of comments below.
> 
> Cheers, Daniel
> 
> >  - Use the small changes to drm_dp_put_port() to add even more error
> >    checking to make misusage of the helpers more obvious. I added this
> >    after having to chase down various use-after-free conditions that
> >    started popping up from the new helpers so no one else has to
> >    troubleshoot that.
> >  - Move some accidental DRM_DEBUG_KMS() calls to DRM_DEBUG_ATOMIC()
> >  - Update documentation again, note that find/release() should both not be
> >    called on the same port in a single atomic check phase (but multiple
> >    calls to one or the other is OK)
> > 
> > Changes since v4:
> >  - Don't skip the atomic checks for VCPI allocations if no new VCPI
> >    allocations happen in a state. This makes the next change I'm about
> >    to list here a lot easier to implement.
> >  - Don't ignore VCPI allocations on destroyed ports, instead ensure that
> >    when ports are destroyed and still have VCPI allocations in the
> >    topology state, the only state changes allowed are releasing said
> >    ports' VCPI. This prevents a state with a mix of VCPI allocations
> >    from destroyed ports, and allocations from valid ports.
> > 
> > Changes since v3:
> >  - Don't release VCPI allocations in the topology state immediately in
> >    drm_dp_atomic_release_vcpi_slots(), instead mark them as 0 and skip
> >    over them in drm_dp_mst_duplicate_state(). This makes it so
> >    drm_dp_atomic_release_vcpi_slots() is still idempotent while also
> >    throwing warnings if the driver messes up it's book keeping and tries
> >    to release VCPI slots on a port that doesn't have any pre-existing
> >    VCPI allocation - danvet
> >  - Change mst_state/state in some debugging messages to "mst state"
> > 
> > Changes since v2:
> >  - Use kmemdup() for duplicating MST state - danvet
> >  - Move port validation out of duplicate state callback - danvet
> >  - Handle looping through MST topology states in
> >    drm_dp_mst_atomic_check() so the driver doesn't have to do it
> >  - Fix documentation in drm_dp_atomic_find_vcpi_slots()
> >  - Move the atomic check for each individual topology state into it's
> >    own function, reduces indenting
> >  - Don't consider "stale" MST ports when calculating the bandwidth
> >    requirements. This is needed because originally we relied on the
> >    state duplication functions to prune any stale ports from the new
> >    state, which would prevent us from incorrectly considering their
> >    bandwidth requirements alongside legitimate new payloads.
> >  - Add function references in drm_dp_atomic_release_vcpi_slots() - danvet
> >  - Annotate atomic VCPI and atomic check functions with __must_check
> >    - danvet
> > 
> > Changes since v1:
> >  - Don't use the now-removed ->atomic_check() for private objects hook,
> >    just give drivers a function to call themselves
> > 
> > Signed-off-by: Lyude Paul <[e-mail]>
> > Cc: Daniel Vetter <[e-mail]>
> > ---
> >  drivers/gpu/drm/drm_dp_mst_topology.c | 294 +++++++++++++++++++++++---
> >  drivers/gpu/drm/i915/intel_display.c  |   4 +
> >  drivers/gpu/drm/i915/intel_dp_mst.c   |  64 +++---
> >  include/drm/drm_dp_mst_helper.h       |  24 ++-
> >  4 files changed, 319 insertions(+), 67 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
> > index 00fbe7a2699d..183d0e832ced 100644
> > --- a/drivers/gpu/drm/drm_dp_mst_topology.c
> > +++ b/drivers/gpu/drm/drm_dp_mst_topology.c
> > @@ -2617,21 +2617,42 @@ static int drm_dp_init_vcpi(struct drm_dp_mst_topology_mgr *mgr,
> >  }
> >  
> >  /**
> > - * drm_dp_atomic_find_vcpi_slots() - Find and add vcpi slots to the state
> > + * drm_dp_atomic_find_vcpi_slots() - Find and add VCPI slots to the state
> >   * @state: global atomic state
> >   * @mgr: MST topology manager for the port
> >   * @port: port to find vcpi slots for
> >   * @pbn: bandwidth required for the mode in PBN
> >   *
> > - * RETURNS:
> > - * Total slots in the atomic state assigned for this port or error
> > + * Allocates VCPI slots to @port, replacing any previous VCPI allocations it
> > + * may have had. Any atomic drivers which support MST must call this function
> > + * in their &drm_encoder_helper_funcs.atomic_check() callback to change the
> > + * current VCPI allocation for the new state, but only when
> > + * &drm_crtc_state.mode_changed or &drm_crtc_state.connectors_changed is set
> > + * to ensure compatibility with userspace applications that still use the
> > + * legacy modesetting UAPI.
> > + *
> > + * Allocations set by this function are not checked against the bandwidth
> > + * restraints of @mgr until the driver calls drm_dp_mst_atomic_check().
> > + *
> > + * Additionally, it is OK to call this function multiple times on the same
> > + * @port as needed. It is not OK however, to call this function and
> > + * drm_dp_atomic_release_vcpi_slots() in the same atomic check phase.
> > + *
> > + * See also:
> > + * drm_dp_atomic_release_vcpi_slots()
> > + * drm_dp_mst_atomic_check()
> > + *
> > + * Returns:
> > + * Total slots in the atomic state assigned for this port, or a negative error
> > + * code if the port no longer exists
> >   */
> >  int drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state *state,
> >  				  struct drm_dp_mst_topology_mgr *mgr,
> >  				  struct drm_dp_mst_port *port, int pbn)
> >  {
> >  	struct drm_dp_mst_topology_state *topology_state;
> > -	int req_slots;
> > +	struct drm_dp_vcpi_allocation *pos, *vcpi = NULL;
> > +	int prev_slots, req_slots, ret;
> >  
> >  	topology_state = drm_atomic_get_mst_topology_state(state, mgr);
> >  	if (IS_ERR(topology_state))
> > @@ -2640,20 +2661,60 @@ int drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state *state,
> >  	port = drm_dp_get_validated_port_ref(mgr, port);
> >  	if (port == NULL)
> >  		return -EINVAL;
> > -	req_slots = DIV_ROUND_UP(pbn, mgr->pbn_div);
> > -	DRM_DEBUG_KMS("vcpi slots req=%d, avail=%d\n",
> > -			req_slots, topology_state->avail_slots);
> >  
> > -	if (req_slots > topology_state->avail_slots) {
> > -		drm_dp_put_port(port);
> > -		return -ENOSPC;
> > +	topology_state->vcpi_allocated = true;
> > +
> > +	/* Find the current allocation for this port, if any */
> > +	list_for_each_entry(pos, &topology_state->vcpis, next) {
> > +		if (pos->port == port) {
> > +			vcpi = pos;
> > +			prev_slots = vcpi->vcpi;
> > +
> > +			/*
> > +			 * This should never happen, unless the driver tries
> > +			 * releasing and allocating the same VCPI allocation,
> > +			 * which is an error
> > +			 */
> > +			if (WARN_ON(!prev_slots)) {
> 
> 
> > +				DRM_ERROR("cannot allocate and release VCPI on [MST PORT:%p] in the same state\n",
> > +					  port);
> > +				return -EINVAL;
> > +			}
> 
> Allowing atomic changes to output state (like disabling it on one crtc,
> and then enabling on the other, thus both releasing and re-allocating the
> vcpi in one go) is kinda the point of atomic. So definitely can't EINVAL
> here.
> 
> Sounds like we need more testcases in igt?
> 
> > +
> > +			break;
> > +		}
> >  	}
> > +	if (!vcpi)
> > +		prev_slots = 0;
> >  
> > -	topology_state->avail_slots -= req_slots;
> > -	DRM_DEBUG_KMS("vcpi slots avail=%d", topology_state->avail_slots);
> > +	req_slots = DIV_ROUND_UP(pbn, mgr->pbn_div);
> > +
> > +	DRM_DEBUG_ATOMIC("[CONNECTOR:%d:%s] [MST PORT:%p] VCPI %d -> %d\n",
> > +			 port->connector->base.id, port->connector->name,
> > +			 port, prev_slots, req_slots);
> > +
> > +	/* Add the new allocation to the state */
> > +	if (!vcpi) {
> > +		vcpi = kzalloc(sizeof(*vcpi), GFP_KERNEL);
> > +		if (!vcpi) {
> > +			ret = -ENOMEM;
> > +			goto out;
> > +		}
> >  
> > +		vcpi->port = drm_dp_get_validated_port_ref(mgr, port);
> > +		if (!vcpi->port) {
> > +			ret = -EINVAL;
> > +			kfree(vcpi);
> > +			goto out;
> > +		}
> > +		list_add(&vcpi->next, &topology_state->vcpis);
> > +	}
> > +	vcpi->vcpi = req_slots;
> > +
> > +	ret = req_slots;
> > +out:
> >  	drm_dp_put_port(port);
> > -	return req_slots;
> > +	return ret;
> >  }
> >  EXPORT_SYMBOL(drm_dp_atomic_find_vcpi_slots);
> >  
> > @@ -2661,31 +2722,66 @@ EXPORT_SYMBOL(drm_dp_atomic_find_vcpi_slots);
> >   * drm_dp_atomic_release_vcpi_slots() - Release allocated vcpi slots
> >   * @state: global atomic state
> >   * @mgr: MST topology manager for the port
> > - * @slots: number of vcpi slots to release
> > + * @port: The port to release the VCPI slots from
> >   *
> > - * RETURNS:
> > - * 0 if @slots were added back to &drm_dp_mst_topology_state->avail_slots or
> > - * negative error code
> > + * Releases any VCPI slots that have been allocated to a port in the atomic
> > + * state. Any atomic drivers which support MST must call this function in
> > + * their &drm_connector_helper_funcs.atomic_check() callback when the
> > + * connector will no longer have VCPI allocated (e.g. because it's CRTC was
> > + * removed) when it had VCPI allocated in the previous atomic state.
> > + *
> > + * It is OK to call this even if @port has been removed from the system.
> > + * Additionally, it is OK to call this function multiple times on the same
> > + * @port as needed. It is not OK however, to call this function and
> > + * drm_dp_atomic_find_vcpi_slots() on the same @port in a single atomic check
> > + * phase.
> > + *
> > + * See also:
> > + * drm_dp_atomic_find_vcpi_slots()
> > + * drm_dp_mst_atomic_check()
> > + *
> > + * Returns:
> > + * 0 if all slots for this port were added back to
> > + * &drm_dp_mst_topology_state.avail_slots or negative error code
> >   */
> >  int drm_dp_atomic_release_vcpi_slots(struct drm_atomic_state *state,
> >  				     struct drm_dp_mst_topology_mgr *mgr,
> > -				     int slots)
> > +				     struct drm_dp_mst_port *port)
> >  {
> >  	struct drm_dp_mst_topology_state *topology_state;
> > +	struct drm_dp_vcpi_allocation *pos;
> > +	bool found = false;
> >  
> >  	topology_state = drm_atomic_get_mst_topology_state(state, mgr);
> >  	if (IS_ERR(topology_state))
> >  		return PTR_ERR(topology_state);
> >  
> > -	/* We cannot rely on port->vcpi.num_slots to update
> > -	 * topology_state->avail_slots as the port may not exist if the parent
> > -	 * branch device was unplugged. This should be fixed by tracking
> > -	 * per-port slot allocation in drm_dp_mst_topology_state instead of
> > -	 * depending on the caller to tell us how many slots to release.
> > -	 */
> > -	topology_state->avail_slots += slots;
> > -	DRM_DEBUG_KMS("vcpi slots released=%d, avail=%d\n",
> > -			slots, topology_state->avail_slots);
> > +	list_for_each_entry(pos, &topology_state->vcpis, next) {
> 
> > +		if (pos->port == port) {
> > +			found = true;
> > +			break;
> > +		}
> > +	}
> > +	if (WARN_ON(!found)) {
> > +		DRM_ERROR("no VCPI for [MST PORT:%p] found in mst state %p\n",
> > +			  port, &topology_state->base);
> > +		return -EINVAL;
> > +	}
> > +
> > +	DRM_DEBUG_ATOMIC("[MST PORT:%p] VCPI %d -> 0\n", port, pos->vcpi);
> > +	if (pos->vcpi) {
> > +		/*
> > +		 * This should be safe-as the previous state will still hold a
> > +		 * reference to this port until we swap states. If it's not,
> > +		 * the driver has a bookkeeping error
> > +		 */
> > +		if (WARN_ON(drm_dp_put_port(port))) {
> 
> The usual way to do this (instead of adding a return value to the normal
> _put function) is something like
> 
> 	kref_put(port, function_that_with_just_a_WARNING_DRM_ERROR)
> 
> avoids the need for patch 2.
> 
> > +			DRM_ERROR("cannot allocate and release VCPI on [MST PORT:%p] in the same state\n",
> > +				  port);
> > +			return -EINVAL;
> > +		}
> > +		pos->vcpi = 0;
> > +	}
> >  
> >  	return 0;
> >  }
> > @@ -3115,15 +3211,45 @@ static void drm_dp_destroy_connector_work(struct work_struct *work)
> >  static struct drm_private_state *
> >  drm_dp_mst_duplicate_state(struct drm_private_obj *obj)
> >  {
> > -	struct drm_dp_mst_topology_state *state;
> > +	struct drm_dp_mst_topology_state *state, *old_state =
> > +		to_dp_mst_topology_state(obj->state);
> > +	struct drm_dp_vcpi_allocation *pos, *vcpi;
> >  
> > -	state = kmemdup(obj->state, sizeof(*state), GFP_KERNEL);
> > +	state = kmemdup(old_state, sizeof(*state), GFP_KERNEL);
> >  	if (!state)
> >  		return NULL;
> >  
> >  	__drm_atomic_helper_private_obj_duplicate_state(obj, &state->base);
> >  
> > +	INIT_LIST_HEAD(&state->vcpis);
> > +	state->vcpi_allocated = false;
> > +
> > +	list_for_each_entry(pos, &old_state->vcpis, next) {
> > +		/* Prune leftover freed VCPI allocations */
> > +		if (!pos->vcpi)
> > +			continue;
> > +
> > +		vcpi = kmemdup(pos, sizeof(*vcpi), GFP_KERNEL);
> > +		if (!vcpi)
> > +			goto fail;
> > +
> > +		/* Take a plain old kref here and don't validate the port, as
> > +		 * it's topology might not even exist anymore
> > +		 */
> > +		kref_get(&vcpi->port->kref);
> > +		list_add(&vcpi->next, &state->vcpis);
> > +	}
> > +
> >  	return &state->base;
> > +
> > +fail:
> > +	list_for_each_entry_safe(pos, vcpi, &state->vcpis, next) {
> > +		drm_dp_put_port(pos->port);
> > +		kfree(pos);
> > +	}
> > +	kfree(state);
> > +
> > +	return NULL;
> >  }
> >  
> >  static void drm_dp_mst_destroy_state(struct drm_private_obj *obj,
> > @@ -3131,14 +3257,116 @@ static void drm_dp_mst_destroy_state(struct drm_private_obj *obj,
> >  {
> >  	struct drm_dp_mst_topology_state *mst_state =
> >  		to_dp_mst_topology_state(state);
> > +	struct drm_dp_vcpi_allocation *pos, *tmp;
> > +
> > +	list_for_each_entry_safe(pos, tmp, &mst_state->vcpis, next) {
> > +		/* We only keep references to ports with non-zero VCPIs */
> > +		if (pos->vcpi)
> > +			drm_dp_put_port(pos->port);
> > +		kfree(pos);
> > +	}
> >  
> >  	kfree(mst_state);
> >  }
> >  
> > -static const struct drm_private_state_funcs mst_state_funcs = {
> > +static inline int
> > +drm_dp_mst_atomic_check_topology_state(struct drm_dp_mst_topology_mgr *mgr,
> > +				       struct drm_dp_mst_topology_state *mst_state)
> > +{
> > +	struct drm_dp_vcpi_allocation *vcpi;
> > +	struct drm_dp_mst_port *port;
> > +	int avail_slots = 63, ret;
> > +
> > +	/* There's no possible scenario where releasing VCPI or keeping it the
> > +	 * same would make the state invalid
> > +	 */
> > +	if (!mst_state->vcpi_allocated) {
> 
> Do we need this? If not I'm always in favour of removing code if it's not
> needed :-)
> 
> > +		DRM_DEBUG_ATOMIC("[MST MGR:%p] state %p allocates no VCPI, check skipped\n",
> > +				 mgr, &mst_state->base);
> > +		return 0;
> > +	}
> > +
> > +	list_for_each_entry(vcpi, &mst_state->vcpis, next) {
> > +		/* Releasing VCPI is always OK-even if the port is gone */
> > +		if (!vcpi->vcpi) {
> > +			DRM_DEBUG_ATOMIC("[MST PORT:%p] releases all VCPI slots\n",
> > +					 vcpi->port);
> > +			continue;
> > +		}
> > +
> > +		port = drm_dp_get_validated_port_ref(mgr, vcpi->port);
> > +		if (!port) {
> > +			DRM_DEBUG_ATOMIC("[MST PORT:%p] is gone but still has VCPI, cannot add new VCPI\n",
> > +					 vcpi->port);
> > +			return -EINVAL;
> > +		}
> 
> For find_vcpi we've already done this. For unplugged vcpi, we can't do
> this here I think - otherwise if you enable 2 mst outputs on the same
> branch, and unplug both, old non-atomic userspace can never ever again
> disable them because it can't disable both. The check (to skip already
> deallocated ports) above isn't enough to handle that.
> 
> > +
> > +		DRM_DEBUG_ATOMIC("[MST PORT:%p] requires %d vcpi slots\n",
> > +				 vcpi->port, vcpi->vcpi);
> > +
> > +		avail_slots -= vcpi->vcpi;
> > +		if (avail_slots < 0) {
> > +			DRM_DEBUG_ATOMIC("[MST PORT:%p] not enough VCPI slots in mst state %p (avail=%d)\n",
> > +					 vcpi->port, mst_state,
> > +					 avail_slots + vcpi->vcpi);
> > +			ret = -ENOSPC;
> > +			goto port_fail;
> > +		}
> > +
> > +		drm_dp_put_port(port);
> > +	}
> > +	DRM_DEBUG_ATOMIC("[MST MGR:%p] mst state %p VCPI avail=%d used=%d\n",
> > +			 mgr, mst_state, avail_slots,
> > +			 63 - avail_slots);
> > +
> > +	return 0;
> > +port_fail:
> > +	drm_dp_put_port(port);
> > +	return ret;
> > +}
> > +
> > +/**
> > + * drm_dp_mst_atomic_check - Check that the new state of an MST topology in an
> > + * atomic update is valid
> > + * @state: Pointer to the new &struct drm_dp_mst_topology_state
> > + *
> > + * Checks the given topology state for an atomic update to ensure that it's
> > + * valid. This includes checking whether there's enough bandwidth to support
> > + * the new VCPI allocations in the atomic update.
> > + *
> > + * Any atomic drivers supporting DP MST must make sure to call this after
> > + * checking the rest of their state in their
> > + * &drm_mode_config_funcs.atomic_check() callback.
> > + *
> > + * See also:
> > + * drm_dp_atomic_find_vcpi_slots()
> > + * drm_dp_atomic_release_vcpi_slots()
> > + *
> > + * Returns:
> > + *
> > + * 0 if the new state is valid, negative error code otherwise.
> > + */
> > +int drm_dp_mst_atomic_check(struct drm_atomic_state *state)
> > +{
> > +	struct drm_dp_mst_topology_mgr *mgr;
> > +	struct drm_dp_mst_topology_state *mst_state;
> > +	int i, ret = 0;
> > +
> > +	for_each_new_mst_mgr_in_state(state, mgr, mst_state, i) {
> > +		ret = drm_dp_mst_atomic_check_topology_state(mgr, mst_state);
> > +		if (ret)
> > +			break;
> > +	}
> > +
> > +	return ret;
> > +}
> > +EXPORT_SYMBOL(drm_dp_mst_atomic_check);
> > +
> > +const struct drm_private_state_funcs drm_dp_mst_topology_state_funcs = {
> >  	.atomic_duplicate_state = drm_dp_mst_duplicate_state,
> >  	.atomic_destroy_state = drm_dp_mst_destroy_state,
> >  };
> > +EXPORT_SYMBOL(drm_dp_mst_topology_state_funcs);
> >  
> >  /**
> >   * drm_atomic_get_mst_topology_state: get MST topology state
> > @@ -3216,13 +3444,11 @@ int drm_dp_mst_topology_mgr_init(struct drm_dp_mst_topology_mgr *mgr,
> >  		return -ENOMEM;
> >  
> >  	mst_state->mgr = mgr;
> > -
> > -	/* max. time slots - one slot for MTP header */
> > -	mst_state->avail_slots = 63;
> > +	INIT_LIST_HEAD(&mst_state->vcpis);
> >  
> >  	drm_atomic_private_obj_init(&mgr->base,
> >  				    &mst_state->base,
> > -				    &mst_state_funcs);
> > +				    &drm_dp_mst_topology_state_funcs);
> >  
> >  	return 0;
> >  }
> > diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
> > index 132e978227fb..8e85e04a5948 100644
> > --- a/drivers/gpu/drm/i915/intel_display.c
> > +++ b/drivers/gpu/drm/i915/intel_display.c
> > @@ -12592,6 +12592,10 @@ static int intel_atomic_check(struct drm_device *dev,
> >  				       "[modeset]" : "[fastset]");
> >  	}
> >  
> > +	ret = drm_dp_mst_atomic_check(state);
> > +	if (ret)
> > +		return ret;
> > +
> >  	if (any_ms) {
> >  		ret = intel_modeset_checks(state);
> >  
> > diff --git a/drivers/gpu/drm/i915/intel_dp_mst.c b/drivers/gpu/drm/i915/intel_dp_mst.c
> > index 4de247ddf05f..be2d1ac7d87d 100644
> > --- a/drivers/gpu/drm/i915/intel_dp_mst.c
> > +++ b/drivers/gpu/drm/i915/intel_dp_mst.c
> > @@ -41,8 +41,13 @@ static bool intel_dp_mst_compute_config(struct intel_encoder *encoder,
> >  	struct drm_connector *connector = conn_state->connector;
> >  	void *port = to_intel_connector(connector)->port;
> >  	struct drm_atomic_state *state = pipe_config->base.state;
> > +	struct drm_crtc *crtc = pipe_config->base.crtc;
> > +	struct drm_crtc_state *old_crtc_state =
> > +		drm_atomic_get_old_crtc_state(state, crtc);
> > +	struct drm_crtc_state *new_crtc_state = &pipe_config->base;
> >  	int bpp;
> > -	int lane_count, slots = 0;
> > +	int lane_count, slots =
> > +		to_intel_crtc_state(old_crtc_state)->dp_m_n.tu;
> >  	const struct drm_display_mode *adjusted_mode = &pipe_config->base.adjusted_mode;
> >  	int mst_pbn;
> >  	bool constant_n = drm_dp_has_quirk(&intel_dp->desc,
> > @@ -77,8 +82,12 @@ static bool intel_dp_mst_compute_config(struct intel_encoder *encoder,
> >  	mst_pbn = drm_dp_calc_pbn_mode(adjusted_mode->crtc_clock, bpp);
> >  	pipe_config->pbn = mst_pbn;
> >  
> > -	/* Zombie connectors can't have VCPI slots */
> > -	if (!drm_connector_is_unregistered(connector)) {
> > +	/* Only change VCPI allocation on actual mode changes, to prevent us
> > +	 * from trying to allocate VCPI to ports that no longer exist when we
> > +	 * may just be trying to disable DPMS on them
> > +	 */
> > +	if (new_crtc_state->mode_changed ||
> > +	    new_crtc_state->connectors_changed) {
> 
> Why exactly is this needed? We only call compute_config on the enocoder
> when enabling an output. Not when disabling one. Similar for the atomic
> helpers and the various encoder callbacks (you have the same change in the
> nouveau code). Did this actually blow up while testing?
> 
> >  		slots = drm_dp_atomic_find_vcpi_slots(state,
> >  						      &intel_dp->mst_mgr,
> >  						      port,
> > @@ -107,36 +116,39 @@ static bool intel_dp_mst_compute_config(struct intel_encoder *encoder,
> >  	return true;
> >  }
> >  
> > -static int intel_dp_mst_atomic_check(struct drm_connector *connector,
> > -		struct drm_connector_state *new_conn_state)
> > +static int
> > +intel_dp_mst_atomic_check(struct drm_connector *connector,
> > +			  struct drm_connector_state *new_conn_state)
> >  {
> > +	struct intel_connector *intel_connector =
> > +		to_intel_connector(connector);
> > +	struct drm_dp_mst_topology_mgr *mgr =
> > +		&intel_connector->mst_port->mst_mgr;
> > +	struct drm_dp_mst_port *port = intel_connector->port;
> >  	struct drm_atomic_state *state = new_conn_state->state;
> > -	struct drm_connector_state *old_conn_state;
> > -	struct drm_crtc *old_crtc;
> > -	struct drm_crtc_state *crtc_state;
> > -	int slots, ret = 0;
> > +	struct drm_connector_state *old_conn_state =
> > +		drm_atomic_get_old_connector_state(state, connector);
> > +	struct drm_crtc *old_crtc = old_conn_state->crtc,
> > +			*new_crtc = new_conn_state->crtc;
> > +	struct drm_crtc_state *old_crtc_state, *new_crtc_state;
> >  
> > -	old_conn_state = drm_atomic_get_old_connector_state(state, connector);
> > -	old_crtc = old_conn_state->crtc;
> 
> Bikeshed: Reworking all the above hides the real changes quite a bit.
> 
> >  	if (!old_crtc)
> > -		return ret;
> > +		return 0;
> >  
> > -	crtc_state = drm_atomic_get_new_crtc_state(state, old_crtc);
> > -	slots = to_intel_crtc_state(crtc_state)->dp_m_n.tu;
> > -	if (drm_atomic_crtc_needs_modeset(crtc_state) && slots > 0) {
> > -		struct drm_dp_mst_topology_mgr *mgr;
> > -		struct drm_encoder *old_encoder;
> > +	old_crtc_state = drm_atomic_get_old_crtc_state(state, old_crtc);
> > +	if (!old_crtc_state ||
> 
> !old_crtc_state would be a framework bug. Just remove that check.
> 
> > +	    !to_intel_crtc_state(old_crtc_state)->dp_m_n.tu)
> > +		return 0;
> 
> Hm, why do we need this? Having a DP port previously enabled with a 0 tu
> sounds like a pretty big driver bug.
> >  
> > -		old_encoder = old_conn_state->best_encoder;
> > -		mgr = &enc_to_mst(old_encoder)->primary->dp.mst_mgr;
> > +	/* If we switched CRTCs, clear the previous one's allocation */
> > +	new_crtc_state = drm_atomic_get_new_crtc_state(state, old_crtc);
> > +	if (new_crtc_state->connectors_changed && !new_crtc_state->enable)
> > +		to_intel_crtc_state(new_crtc_state)->dp_m_n.tu = 0;
> 
> No idea why we need to clear this here? Shouldn't we just clear this
> unconditionally if we release the allocation.
> 
> Also naming convention, I'd call this old_crtc_new_state, and maybe
> old_crtc_old_state. To make it clear these are the old/new states for
> old_crtc. Instead of old/new state for the new crtc.
> 
> >  
> > -		ret = drm_dp_atomic_release_vcpi_slots(state, mgr, slots);
> > -		if (ret)
> > -			DRM_DEBUG_KMS("failed releasing %d vcpi slots:%d\n", slots, ret);
> > -		else
> > -			to_intel_crtc_state(crtc_state)->dp_m_n.tu = 0;
> > -	}
> > -	return ret;
> > +	if (new_crtc)
> > +		return 0;
> 
> Not sure why we shouldn't release this? If we handle this in helpers, it
> would remove piles of special casing code here I think. Plus remove
> special code in the hlpers.
> 
> 
> > +
> > +	return drm_dp_atomic_release_vcpi_slots(state, mgr, port);
> >  }
> >  
> >  static void intel_mst_disable_dp(struct intel_encoder *encoder,
> > diff --git a/include/drm/drm_dp_mst_helper.h b/include/drm/drm_dp_mst_helper.h
> > index 3faceb66f5cb..2392e12b6bce 100644
> > --- a/include/drm/drm_dp_mst_helper.h
> > +++ b/include/drm/drm_dp_mst_helper.h
> > @@ -406,9 +406,16 @@ struct drm_dp_payload {
> >  
> >  #define to_dp_mst_topology_state(x) container_of(x, struct drm_dp_mst_topology_state, base)
> >  
> > +struct drm_dp_vcpi_allocation {
> 
> I think a comment here that this pointer holds a full reference (even past
> when the connector is unplugged) would be good. Otherwise the loop in
> release_vcpi would be buggy.
> 
> > +	struct drm_dp_mst_port *port;
> > +	int vcpi;
> > +	struct list_head next;
> > +};
> > +
> >  struct drm_dp_mst_topology_state {
> >  	struct drm_private_state base;
> > -	int avail_slots;
> > +	struct list_head vcpis;
> > +	u8 vcpi_allocated;
> >  	struct drm_dp_mst_topology_mgr *mgr;
> >  };
> >  
> > @@ -619,14 +626,17 @@ void drm_dp_mst_topology_mgr_suspend(struct drm_dp_mst_topology_mgr *mgr);
> >  int drm_dp_mst_topology_mgr_resume(struct drm_dp_mst_topology_mgr *mgr);
> >  struct drm_dp_mst_topology_state *drm_atomic_get_mst_topology_state(struct drm_atomic_state *state,
> >  								    struct drm_dp_mst_topology_mgr *mgr);
> > -int drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state *state,
> > -				  struct drm_dp_mst_topology_mgr *mgr,
> > -				  struct drm_dp_mst_port *port, int pbn);
> > -int drm_dp_atomic_release_vcpi_slots(struct drm_atomic_state *state,
> > -				     struct drm_dp_mst_topology_mgr *mgr,
> > -				     int slots);
> > +int __must_check
> > +drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state *state,
> > +			      struct drm_dp_mst_topology_mgr *mgr,
> > +			      struct drm_dp_mst_port *port, int pbn);
> > +int __must_check
> > +drm_dp_atomic_release_vcpi_slots(struct drm_atomic_state *state,
> > +				 struct drm_dp_mst_topology_mgr *mgr,
> > +				 struct drm_dp_mst_port *port);
> >  int drm_dp_send_power_updown_phy(struct drm_dp_mst_topology_mgr *mgr,
> >  				 struct drm_dp_mst_port *port, bool power_up);
> > +int __must_check drm_dp_mst_atomic_check(struct drm_atomic_state *state);
> >  
> >  extern const struct drm_private_state_funcs drm_dp_mst_topology_state_funcs;
> >  
> > -- 
> > 2.19.1
> > 
> 
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch

Lyude Paul Nov. 26, 2018, 9:36 p.m. UTC | #3

On Mon, 2018-11-26 at 22:04 +0100, Daniel Vetter wrote:
> On Thu, Nov 15, 2018 at 07:50:05PM -0500, Lyude Paul wrote:
> > There has been a TODO waiting for quite a long time in
> > drm_dp_mst_topology.c:
> > 
> > 	/* We cannot rely on port->vcpi.num_slots to update
> > 	 * topology_state->avail_slots as the port may not exist if the parent
> > 	 * branch device was unplugged. This should be fixed by tracking
> > 	 * per-port slot allocation in drm_dp_mst_topology_state instead of
> > 	 * depending on the caller to tell us how many slots to release.
> > 	 */
> > 
> > That's not the only reason we should fix this: forcing the driver to
> > track the VCPI allocations throughout a state's atomic check is
> > error prone, because it means that extra care has to be taken with the
> > order that drm_dp_atomic_find_vcpi_slots() and
> > drm_dp_atomic_release_vcpi_slots() are called in in order to ensure
> > idempotency. Currently the only driver actually using these helpers,
> > i915, doesn't even do this correctly: multiple ->best_encoder() checks
> > with i915's current implementation would not be idempotent and would
> > over-allocate VCPI slots, something I learned trying to implement
> > fallback retraining in MST.
> > 
> > So: simplify this whole mess, and teach drm_dp_atomic_find_vcpi_slots()
> > and drm_dp_atomic_release_vcpi_slots() to track the VCPI allocations for
> > each port. This allows us to ensure idempotency without having to rely
> > on the driver as much. Additionally: the driver doesn't need to do any
> > kind of VCPI slot tracking anymore if it doesn't need it for it's own
> > internal state.
> > 
> > Additionally; this adds a new drm_dp_mst_atomic_check() helper which
> > must be used by atomic drivers to perform validity checks for the new
> > VCPI allocations incurred by a state.
> > 
> > Also: update the documentation and make it more obvious that these
> > /must/ be called by /all/ atomic drivers supporting MST.
> > 
> > Changes since v6:
> >  - Keep a kref to all of the ports we have allocations on. This required
> >    a good bit of changing to when we call drm_dp_find_vcpi_slots(),
> >    mainly that we need to ensure that we only redo VCPI allocations on
> >    actual mode or CRTC changes, not crtc_state->active changes.
> >    Additionally, we no longer take the registration of the DRM connector
> >    for each port into account because so long as we have a kref to the
> >    port in the new or previous atomic state, the connector will stay
> >    registered.
> 
> I write an entire pile of small nitpits (still included most of them
> below), until I realized this here wont work. Delaying the call to destroy
> the connector (well, unregister it really) wreaks the design we've come up
> with thus far, resulting in most of my comments here.
> 
> Instead, all we need to do is delay the kfree(port) at the bottom of
> drm_dp_destroy_port(). The vcpi allocation structure _only_ needs the
> pointer value to stay valid, as a lookup key. It doesn't care at all about
> anything actually stored in there. So the only thing we need to delay is
> the kfree. I think the simplest way to achieve this is to add a 2nd kref
> (port->kfree_ref or something like that), with on reference held by the
> port itself (released in drm_dp_destroy_port), and the other one held
> as-needed by the vcpi allocation lists.
> 
> I think if we go with this design instead of retrofitting a semantic
> change of the port lifetime itself, all the complications I complain about
> below should disappear.
> 
> Piles of comments below.
> 
> Cheers, Daniel
> 
> >  - Use the small changes to drm_dp_put_port() to add even more error
> >    checking to make misusage of the helpers more obvious. I added this
> >    after having to chase down various use-after-free conditions that
> >    started popping up from the new helpers so no one else has to
> >    troubleshoot that.
> >  - Move some accidental DRM_DEBUG_KMS() calls to DRM_DEBUG_ATOMIC()
> >  - Update documentation again, note that find/release() should both not be
> >    called on the same port in a single atomic check phase (but multiple
> >    calls to one or the other is OK)
> > 
> > Changes since v4:
> >  - Don't skip the atomic checks for VCPI allocations if no new VCPI
> >    allocations happen in a state. This makes the next change I'm about
> >    to list here a lot easier to implement.
> >  - Don't ignore VCPI allocations on destroyed ports, instead ensure that
> >    when ports are destroyed and still have VCPI allocations in the
> >    topology state, the only state changes allowed are releasing said
> >    ports' VCPI. This prevents a state with a mix of VCPI allocations
> >    from destroyed ports, and allocations from valid ports.
> > 
> > Changes since v3:
> >  - Don't release VCPI allocations in the topology state immediately in
> >    drm_dp_atomic_release_vcpi_slots(), instead mark them as 0 and skip
> >    over them in drm_dp_mst_duplicate_state(). This makes it so
> >    drm_dp_atomic_release_vcpi_slots() is still idempotent while also
> >    throwing warnings if the driver messes up it's book keeping and tries
> >    to release VCPI slots on a port that doesn't have any pre-existing
> >    VCPI allocation - danvet
> >  - Change mst_state/state in some debugging messages to "mst state"
> > 
> > Changes since v2:
> >  - Use kmemdup() for duplicating MST state - danvet
> >  - Move port validation out of duplicate state callback - danvet
> >  - Handle looping through MST topology states in
> >    drm_dp_mst_atomic_check() so the driver doesn't have to do it
> >  - Fix documentation in drm_dp_atomic_find_vcpi_slots()
> >  - Move the atomic check for each individual topology state into it's
> >    own function, reduces indenting
> >  - Don't consider "stale" MST ports when calculating the bandwidth
> >    requirements. This is needed because originally we relied on the
> >    state duplication functions to prune any stale ports from the new
> >    state, which would prevent us from incorrectly considering their
> >    bandwidth requirements alongside legitimate new payloads.
> >  - Add function references in drm_dp_atomic_release_vcpi_slots() - danvet
> >  - Annotate atomic VCPI and atomic check functions with __must_check
> >    - danvet
> > 
> > Changes since v1:
> >  - Don't use the now-removed ->atomic_check() for private objects hook,
> >    just give drivers a function to call themselves
> > 
> > Signed-off-by: Lyude Paul <[e-mail]>
> > Cc: Daniel Vetter <[e-mail]>
> > ---
> >  drivers/gpu/drm/drm_dp_mst_topology.c | 294 +++++++++++++++++++++++---
> >  drivers/gpu/drm/i915/intel_display.c  |   4 +
> >  drivers/gpu/drm/i915/intel_dp_mst.c   |  64 +++---
> >  include/drm/drm_dp_mst_helper.h       |  24 ++-
> >  4 files changed, 319 insertions(+), 67 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c
> > b/drivers/gpu/drm/drm_dp_mst_topology.c
> > index 00fbe7a2699d..183d0e832ced 100644
> > --- a/drivers/gpu/drm/drm_dp_mst_topology.c
> > +++ b/drivers/gpu/drm/drm_dp_mst_topology.c
> > @@ -2617,21 +2617,42 @@ static int drm_dp_init_vcpi(struct
> > drm_dp_mst_topology_mgr *mgr,
> >  }
> >  
> >  /**
> > - * drm_dp_atomic_find_vcpi_slots() - Find and add vcpi slots to the state
> > + * drm_dp_atomic_find_vcpi_slots() - Find and add VCPI slots to the state
> >   * @state: global atomic state
> >   * @mgr: MST topology manager for the port
> >   * @port: port to find vcpi slots for
> >   * @pbn: bandwidth required for the mode in PBN
> >   *
> > - * RETURNS:
> > - * Total slots in the atomic state assigned for this port or error
> > + * Allocates VCPI slots to @port, replacing any previous VCPI allocations
> > it
> > + * may have had. Any atomic drivers which support MST must call this
> > function
> > + * in their &drm_encoder_helper_funcs.atomic_check() callback to change
> > the
> > + * current VCPI allocation for the new state, but only when
> > + * &drm_crtc_state.mode_changed or &drm_crtc_state.connectors_changed is
> > set
> > + * to ensure compatibility with userspace applications that still use the
> > + * legacy modesetting UAPI.
> > + *
> > + * Allocations set by this function are not checked against the bandwidth
> > + * restraints of @mgr until the driver calls drm_dp_mst_atomic_check().
> > + *
> > + * Additionally, it is OK to call this function multiple times on the
> > same
> > + * @port as needed. It is not OK however, to call this function and
> > + * drm_dp_atomic_release_vcpi_slots() in the same atomic check phase.
> > + *
> > + * See also:
> > + * drm_dp_atomic_release_vcpi_slots()
> > + * drm_dp_mst_atomic_check()
> > + *
> > + * Returns:
> > + * Total slots in the atomic state assigned for this port, or a negative
> > error
> > + * code if the port no longer exists
> >   */
> >  int drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state *state,
> >  				  struct drm_dp_mst_topology_mgr *mgr,
> >  				  struct drm_dp_mst_port *port, int pbn)
> >  {
> >  	struct drm_dp_mst_topology_state *topology_state;
> > -	int req_slots;
> > +	struct drm_dp_vcpi_allocation *pos, *vcpi = NULL;
> > +	int prev_slots, req_slots, ret;
> >  
> >  	topology_state = drm_atomic_get_mst_topology_state(state, mgr);
> >  	if (IS_ERR(topology_state))
> > @@ -2640,20 +2661,60 @@ int drm_dp_atomic_find_vcpi_slots(struct
> > drm_atomic_state *state,
> >  	port = drm_dp_get_validated_port_ref(mgr, port);
> >  	if (port == NULL)
> >  		return -EINVAL;
> > -	req_slots = DIV_ROUND_UP(pbn, mgr->pbn_div);
> > -	DRM_DEBUG_KMS("vcpi slots req=%d, avail=%d\n",
> > -			req_slots, topology_state->avail_slots);
> >  
> > -	if (req_slots > topology_state->avail_slots) {
> > -		drm_dp_put_port(port);
> > -		return -ENOSPC;
> > +	topology_state->vcpi_allocated = true;
> > +
> > +	/* Find the current allocation for this port, if any */
> > +	list_for_each_entry(pos, &topology_state->vcpis, next) {
> > +		if (pos->port == port) {
> > +			vcpi = pos;
> > +			prev_slots = vcpi->vcpi;
> > +
> > +			/*
> > +			 * This should never happen, unless the driver tries
> > +			 * releasing and allocating the same VCPI allocation,
> > +			 * which is an error
> > +			 */
> > +			if (WARN_ON(!prev_slots)) {
> > +				DRM_ERROR("cannot allocate and release VCPI on
> > [MST PORT:%p] in the same state\n",
> > +					  port);
> > +				return -EINVAL;
> > +			}
> 
> Allowing atomic changes to output state (like disabling it on one crtc,
> and then enabling on the other, thus both releasing and re-allocating the
> vcpi in one go) is kinda the point of atomic. So definitely can't EINVAL
> here.
Actually-so long the original assertion we had of not allowing "stale" (e.g.
VCPIs from now-destroyed topologies) VCPI allocations and real VCPI
allocations in the same atomic state is held true, this should be fine.

So: (and this answers the question below as well), the way this is supposed to
work is that we just skip the whole VCPI validation sequence unless part of
the state actually tried to add VCPI. This means userland can still disable
displays one-by-one or set them as inactive, so long as it doesn't try to
enable new MST displays until the point at which it's disabled everything
leftover from the previous topology - since disabling displays won't allocate
VCPI and thus skips the port validation checks here.

> 
> Sounds like we need more testcases in igt?
> 
> > +
> > +			break;
> > +		}
> >  	}
> > +	if (!vcpi)
> > +		prev_slots = 0;
> >  
> > -	topology_state->avail_slots -= req_slots;
> > -	DRM_DEBUG_KMS("vcpi slots avail=%d", topology_state->avail_slots);
> > +	req_slots = DIV_ROUND_UP(pbn, mgr->pbn_div);
> > +
> > +	DRM_DEBUG_ATOMIC("[CONNECTOR:%d:%s] [MST PORT:%p] VCPI %d -> %d\n",
> > +			 port->connector->base.id, port->connector->name,
> > +			 port, prev_slots, req_slots);
> > +
> > +	/* Add the new allocation to the state */
> > +	if (!vcpi) {
> > +		vcpi = kzalloc(sizeof(*vcpi), GFP_KERNEL);
> > +		if (!vcpi) {
> > +			ret = -ENOMEM;
> > +			goto out;
> > +		}
> >  
> > +		vcpi->port = drm_dp_get_validated_port_ref(mgr, port);
> > +		if (!vcpi->port) {
> > +			ret = -EINVAL;
> > +			kfree(vcpi);
> > +			goto out;
> > +		}
> > +		list_add(&vcpi->next, &topology_state->vcpis);
> > +	}
> > +	vcpi->vcpi = req_slots;
> > +
> > +	ret = req_slots;
> > +out:
> >  	drm_dp_put_port(port);
> > -	return req_slots;
> > +	return ret;
> >  }
> >  EXPORT_SYMBOL(drm_dp_atomic_find_vcpi_slots);
> >  
> > @@ -2661,31 +2722,66 @@ EXPORT_SYMBOL(drm_dp_atomic_find_vcpi_slots);
> >   * drm_dp_atomic_release_vcpi_slots() - Release allocated vcpi slots
> >   * @state: global atomic state
> >   * @mgr: MST topology manager for the port
> > - * @slots: number of vcpi slots to release
> > + * @port: The port to release the VCPI slots from
> >   *
> > - * RETURNS:
> > - * 0 if @slots were added back to &drm_dp_mst_topology_state->avail_slots 
> > or
> > - * negative error code
> > + * Releases any VCPI slots that have been allocated to a port in the
> > atomic
> > + * state. Any atomic drivers which support MST must call this function in
> > + * their &drm_connector_helper_funcs.atomic_check() callback when the
> > + * connector will no longer have VCPI allocated (e.g. because it's CRTC
> > was
> > + * removed) when it had VCPI allocated in the previous atomic state.
> > + *
> > + * It is OK to call this even if @port has been removed from the system.
> > + * Additionally, it is OK to call this function multiple times on the
> > same
> > + * @port as needed. It is not OK however, to call this function and
> > + * drm_dp_atomic_find_vcpi_slots() on the same @port in a single atomic
> > check
> > + * phase.
> > + *
> > + * See also:
> > + * drm_dp_atomic_find_vcpi_slots()
> > + * drm_dp_mst_atomic_check()
> > + *
> > + * Returns:
> > + * 0 if all slots for this port were added back to
> > + * &drm_dp_mst_topology_state.avail_slots or negative error code
> >   */
> >  int drm_dp_atomic_release_vcpi_slots(struct drm_atomic_state *state,
> >  				     struct drm_dp_mst_topology_mgr *mgr,
> > -				     int slots)
> > +				     struct drm_dp_mst_port *port)
> >  {
> >  	struct drm_dp_mst_topology_state *topology_state;
> > +	struct drm_dp_vcpi_allocation *pos;
> > +	bool found = false;
> >  
> >  	topology_state = drm_atomic_get_mst_topology_state(state, mgr);
> >  	if (IS_ERR(topology_state))
> >  		return PTR_ERR(topology_state);
> >  
> > -	/* We cannot rely on port->vcpi.num_slots to update
> > -	 * topology_state->avail_slots as the port may not exist if the parent
> > -	 * branch device was unplugged. This should be fixed by tracking
> > -	 * per-port slot allocation in drm_dp_mst_topology_state instead of
> > -	 * depending on the caller to tell us how many slots to release.
> > -	 */
> > -	topology_state->avail_slots += slots;
> > -	DRM_DEBUG_KMS("vcpi slots released=%d, avail=%d\n",
> > -			slots, topology_state->avail_slots);
> > +	list_for_each_entry(pos, &topology_state->vcpis, next) {
> > +		if (pos->port == port) {
> > +			found = true;
> > +			break;
> > +		}
> > +	}
> > +	if (WARN_ON(!found)) {
> > +		DRM_ERROR("no VCPI for [MST PORT:%p] found in mst state %p\n",
> > +			  port, &topology_state->base);
> > +		return -EINVAL;
> > +	}
> > +
> > +	DRM_DEBUG_ATOMIC("[MST PORT:%p] VCPI %d -> 0\n", port, pos->vcpi);
> > +	if (pos->vcpi) {
> > +		/*
> > +		 * This should be safe-as the previous state will still hold a
> > +		 * reference to this port until we swap states. If it's not,
> > +		 * the driver has a bookkeeping error
> > +		 */
> > +		if (WARN_ON(drm_dp_put_port(port))) {
> 
> The usual way to do this (instead of adding a return value to the normal
> _put function) is something like
> 
> 	kref_put(port, function_that_with_just_a_WARNING_DRM_ERROR)
> 
> avoids the need for patch 2.
> 
> > +			DRM_ERROR("cannot allocate and release VCPI on [MST
> > PORT:%p] in the same state\n",
> > +				  port);
> > +			return -EINVAL;
> > +		}
> > +		pos->vcpi = 0;
> > +	}
> >  
> >  	return 0;
> >  }
> > @@ -3115,15 +3211,45 @@ static void drm_dp_destroy_connector_work(struct
> > work_struct *work)
> >  static struct drm_private_state *
> >  drm_dp_mst_duplicate_state(struct drm_private_obj *obj)
> >  {
> > -	struct drm_dp_mst_topology_state *state;
> > +	struct drm_dp_mst_topology_state *state, *old_state =
> > +		to_dp_mst_topology_state(obj->state);
> > +	struct drm_dp_vcpi_allocation *pos, *vcpi;
> >  
> > -	state = kmemdup(obj->state, sizeof(*state), GFP_KERNEL);
> > +	state = kmemdup(old_state, sizeof(*state), GFP_KERNEL);
> >  	if (!state)
> >  		return NULL;
> >  
> >  	__drm_atomic_helper_private_obj_duplicate_state(obj, &state->base);
> >  
> > +	INIT_LIST_HEAD(&state->vcpis);
> > +	state->vcpi_allocated = false;
> > +
> > +	list_for_each_entry(pos, &old_state->vcpis, next) {
> > +		/* Prune leftover freed VCPI allocations */
> > +		if (!pos->vcpi)
> > +			continue;
> > +
> > +		vcpi = kmemdup(pos, sizeof(*vcpi), GFP_KERNEL);
> > +		if (!vcpi)
> > +			goto fail;
> > +
> > +		/* Take a plain old kref here and don't validate the port, as
> > +		 * it's topology might not even exist anymore
> > +		 */
> > +		kref_get(&vcpi->port->kref);
> > +		list_add(&vcpi->next, &state->vcpis);
> > +	}
> > +
> >  	return &state->base;
> > +
> > +fail:
> > +	list_for_each_entry_safe(pos, vcpi, &state->vcpis, next) {
> > +		drm_dp_put_port(pos->port);
> > +		kfree(pos);
> > +	}
> > +	kfree(state);
> > +
> > +	return NULL;
> >  }
> >  
> >  static void drm_dp_mst_destroy_state(struct drm_private_obj *obj,
> > @@ -3131,14 +3257,116 @@ static void drm_dp_mst_destroy_state(struct
> > drm_private_obj *obj,
> >  {
> >  	struct drm_dp_mst_topology_state *mst_state =
> >  		to_dp_mst_topology_state(state);
> > +	struct drm_dp_vcpi_allocation *pos, *tmp;
> > +
> > +	list_for_each_entry_safe(pos, tmp, &mst_state->vcpis, next) {
> > +		/* We only keep references to ports with non-zero VCPIs */
> > +		if (pos->vcpi)
> > +			drm_dp_put_port(pos->port);
> > +		kfree(pos);
> > +	}
> >  
> >  	kfree(mst_state);
> >  }
> >  
> > -static const struct drm_private_state_funcs mst_state_funcs = {
> > +static inline int
> > +drm_dp_mst_atomic_check_topology_state(struct drm_dp_mst_topology_mgr
> > *mgr,
> > +				       struct drm_dp_mst_topology_state
> > *mst_state)
> > +{
> > +	struct drm_dp_vcpi_allocation *vcpi;
> > +	struct drm_dp_mst_port *port;
> > +	int avail_slots = 63, ret;
> > +
> > +	/* There's no possible scenario where releasing VCPI or keeping it the
> > +	 * same would make the state invalid
> > +	 */
> > +	if (!mst_state->vcpi_allocated) {
> 
> Do we need this? If not I'm always in favour of removing code if it's not
> needed :-)
answered by my explanation above ^
> 
> > +		DRM_DEBUG_ATOMIC("[MST MGR:%p] state %p allocates no VCPI,
> > check skipped\n",
> > +				 mgr, &mst_state->base);
> > +		return 0;
> > +	}
> > +
> > +	list_for_each_entry(vcpi, &mst_state->vcpis, next) {
> > +		/* Releasing VCPI is always OK-even if the port is gone */
> > +		if (!vcpi->vcpi) {
> > +			DRM_DEBUG_ATOMIC("[MST PORT:%p] releases all VCPI
> > slots\n",
> > +					 vcpi->port);
> > +			continue;
> > +		}
> > +
> > +		port = drm_dp_get_validated_port_ref(mgr, vcpi->port);
> > +		if (!port) {
> > +			DRM_DEBUG_ATOMIC("[MST PORT:%p] is gone but still has
> > VCPI, cannot add new VCPI\n",
> > +					 vcpi->port);
> > +			return -EINVAL;
> > +		}
> 
> For find_vcpi we've already done this. For unplugged vcpi, we can't do
> this here I think - otherwise if you enable 2 mst outputs on the same
> branch, and unplug both, old non-atomic userspace can never ever again
> disable them because it can't disable both. The check (to skip already
> deallocated ports) above isn't enough to handle that.

answered by my explanation above ^
> 
> > +
> > +		DRM_DEBUG_ATOMIC("[MST PORT:%p] requires %d vcpi slots\n",
> > +				 vcpi->port, vcpi->vcpi);
> > +
> > +		avail_slots -= vcpi->vcpi;
> > +		if (avail_slots < 0) {
> > +			DRM_DEBUG_ATOMIC("[MST PORT:%p] not enough VCPI slots
> > in mst state %p (avail=%d)\n",
> > +					 vcpi->port, mst_state,
> > +					 avail_slots + vcpi->vcpi);
> > +			ret = -ENOSPC;
> > +			goto port_fail;
> > +		}
> > +
> > +		drm_dp_put_port(port);
> > +	}
> > +	DRM_DEBUG_ATOMIC("[MST MGR:%p] mst state %p VCPI avail=%d used=%d\n",
> > +			 mgr, mst_state, avail_slots,
> > +			 63 - avail_slots);
> > +
> > +	return 0;
> > +port_fail:
> > +	drm_dp_put_port(port);
> > +	return ret;
> > +}
> > +
> > +/**
> > + * drm_dp_mst_atomic_check - Check that the new state of an MST topology
> > in an
> > + * atomic update is valid
> > + * @state: Pointer to the new &struct drm_dp_mst_topology_state
> > + *
> > + * Checks the given topology state for an atomic update to ensure that
> > it's
> > + * valid. This includes checking whether there's enough bandwidth to
> > support
> > + * the new VCPI allocations in the atomic update.
> > + *
> > + * Any atomic drivers supporting DP MST must make sure to call this after
> > + * checking the rest of their state in their
> > + * &drm_mode_config_funcs.atomic_check() callback.
> > + *
> > + * See also:
> > + * drm_dp_atomic_find_vcpi_slots()
> > + * drm_dp_atomic_release_vcpi_slots()
> > + *
> > + * Returns:
> > + *
> > + * 0 if the new state is valid, negative error code otherwise.
> > + */
> > +int drm_dp_mst_atomic_check(struct drm_atomic_state *state)
> > +{
> > +	struct drm_dp_mst_topology_mgr *mgr;
> > +	struct drm_dp_mst_topology_state *mst_state;
> > +	int i, ret = 0;
> > +
> > +	for_each_new_mst_mgr_in_state(state, mgr, mst_state, i) {
> > +		ret = drm_dp_mst_atomic_check_topology_state(mgr, mst_state);
> > +		if (ret)
> > +			break;
> > +	}
> > +
> > +	return ret;
> > +}
> > +EXPORT_SYMBOL(drm_dp_mst_atomic_check);
> > +
> > +const struct drm_private_state_funcs drm_dp_mst_topology_state_funcs = {
> >  	.atomic_duplicate_state = drm_dp_mst_duplicate_state,
> >  	.atomic_destroy_state = drm_dp_mst_destroy_state,
> >  };
> > +EXPORT_SYMBOL(drm_dp_mst_topology_state_funcs);
> >  
> >  /**
> >   * drm_atomic_get_mst_topology_state: get MST topology state
> > @@ -3216,13 +3444,11 @@ int drm_dp_mst_topology_mgr_init(struct
> > drm_dp_mst_topology_mgr *mgr,
> >  		return -ENOMEM;
> >  
> >  	mst_state->mgr = mgr;
> > -
> > -	/* max. time slots - one slot for MTP header */
> > -	mst_state->avail_slots = 63;
> > +	INIT_LIST_HEAD(&mst_state->vcpis);
> >  
> >  	drm_atomic_private_obj_init(&mgr->base,
> >  				    &mst_state->base,
> > -				    &mst_state_funcs);
> > +				    &drm_dp_mst_topology_state_funcs);
> >  
> >  	return 0;
> >  }
> > diff --git a/drivers/gpu/drm/i915/intel_display.c
> > b/drivers/gpu/drm/i915/intel_display.c
> > index 132e978227fb..8e85e04a5948 100644
> > --- a/drivers/gpu/drm/i915/intel_display.c
> > +++ b/drivers/gpu/drm/i915/intel_display.c
> > @@ -12592,6 +12592,10 @@ static int intel_atomic_check(struct drm_device
> > *dev,
> >  				       "[modeset]" : "[fastset]");
> >  	}
> >  
> > +	ret = drm_dp_mst_atomic_check(state);
> > +	if (ret)
> > +		return ret;
> > +
> >  	if (any_ms) {
> >  		ret = intel_modeset_checks(state);
> >  
> > diff --git a/drivers/gpu/drm/i915/intel_dp_mst.c
> > b/drivers/gpu/drm/i915/intel_dp_mst.c
> > index 4de247ddf05f..be2d1ac7d87d 100644
> > --- a/drivers/gpu/drm/i915/intel_dp_mst.c
> > +++ b/drivers/gpu/drm/i915/intel_dp_mst.c
> > @@ -41,8 +41,13 @@ static bool intel_dp_mst_compute_config(struct
> > intel_encoder *encoder,
> >  	struct drm_connector *connector = conn_state->connector;
> >  	void *port = to_intel_connector(connector)->port;
> >  	struct drm_atomic_state *state = pipe_config->base.state;
> > +	struct drm_crtc *crtc = pipe_config->base.crtc;
> > +	struct drm_crtc_state *old_crtc_state =
> > +		drm_atomic_get_old_crtc_state(state, crtc);
> > +	struct drm_crtc_state *new_crtc_state = &pipe_config->base;
> >  	int bpp;
> > -	int lane_count, slots = 0;
> > +	int lane_count, slots =
> > +		to_intel_crtc_state(old_crtc_state)->dp_m_n.tu;
> >  	const struct drm_display_mode *adjusted_mode = &pipe_config-
> > >base.adjusted_mode;
> >  	int mst_pbn;
> >  	bool constant_n = drm_dp_has_quirk(&intel_dp->desc,
> > @@ -77,8 +82,12 @@ static bool intel_dp_mst_compute_config(struct
> > intel_encoder *encoder,
> >  	mst_pbn = drm_dp_calc_pbn_mode(adjusted_mode->crtc_clock, bpp);
> >  	pipe_config->pbn = mst_pbn;
> >  
> > -	/* Zombie connectors can't have VCPI slots */
> > -	if (!drm_connector_is_unregistered(connector)) {
> > +	/* Only change VCPI allocation on actual mode changes, to prevent us
> > +	 * from trying to allocate VCPI to ports that no longer exist when we
> > +	 * may just be trying to disable DPMS on them
> > +	 */
> > +	if (new_crtc_state->mode_changed ||
> > +	    new_crtc_state->connectors_changed) {
> 
> Why exactly is this needed? We only call compute_config on the enocoder
> when enabling an output. Not when disabling one. Similar for the atomic
> helpers and the various encoder callbacks (you have the same change in the
> nouveau code). Did this actually blow up while testing?

yes-it looked like we were calling compute_config on simple DPMS on->off
chnges as well, this ended up making it so we wouldn't try changing the VCPI
allocation on such commits.

ACK on all of the other comments here.
> 
> >  		slots = drm_dp_atomic_find_vcpi_slots(state,
> >  						      &intel_dp->mst_mgr,
> >  						      port,
> > @@ -107,36 +116,39 @@ static bool intel_dp_mst_compute_config(struct
> > intel_encoder *encoder,
> >  	return true;
> >  }
> >  
> > -static int intel_dp_mst_atomic_check(struct drm_connector *connector,
> > -		struct drm_connector_state *new_conn_state)
> > +static int
> > +intel_dp_mst_atomic_check(struct drm_connector *connector,
> > +			  struct drm_connector_state *new_conn_state)
> >  {
> > +	struct intel_connector *intel_connector =
> > +		to_intel_connector(connector);
> > +	struct drm_dp_mst_topology_mgr *mgr =
> > +		&intel_connector->mst_port->mst_mgr;
> > +	struct drm_dp_mst_port *port = intel_connector->port;
> >  	struct drm_atomic_state *state = new_conn_state->state;
> > -	struct drm_connector_state *old_conn_state;
> > -	struct drm_crtc *old_crtc;
> > -	struct drm_crtc_state *crtc_state;
> > -	int slots, ret = 0;
> > +	struct drm_connector_state *old_conn_state =
> > +		drm_atomic_get_old_connector_state(state, connector);
> > +	struct drm_crtc *old_crtc = old_conn_state->crtc,
> > +			*new_crtc = new_conn_state->crtc;
> > +	struct drm_crtc_state *old_crtc_state, *new_crtc_state;
> >  
> > -	old_conn_state = drm_atomic_get_old_connector_state(state, connector);
> > -	old_crtc = old_conn_state->crtc;
> 
> Bikeshed: Reworking all the above hides the real changes quite a bit.
> 
> >  	if (!old_crtc)
> > -		return ret;
> > +		return 0;
> >  
> > -	crtc_state = drm_atomic_get_new_crtc_state(state, old_crtc);
> > -	slots = to_intel_crtc_state(crtc_state)->dp_m_n.tu;
> > -	if (drm_atomic_crtc_needs_modeset(crtc_state) && slots > 0) {
> > -		struct drm_dp_mst_topology_mgr *mgr;
> > -		struct drm_encoder *old_encoder;
> > +	old_crtc_state = drm_atomic_get_old_crtc_state(state, old_crtc);
> > +	if (!old_crtc_state ||
> 
> !old_crtc_state would be a framework bug. Just remove that check.
> 
> > +	    !to_intel_crtc_state(old_crtc_state)->dp_m_n.tu)
> > +		return 0;
> 
> Hm, why do we need this? Having a DP port previously enabled with a 0 tu
> sounds like a pretty big driver bug.
> >  
> > -		old_encoder = old_conn_state->best_encoder;
> > -		mgr = &enc_to_mst(old_encoder)->primary->dp.mst_mgr;
> > +	/* If we switched CRTCs, clear the previous one's allocation */
> > +	new_crtc_state = drm_atomic_get_new_crtc_state(state, old_crtc);
> > +	if (new_crtc_state->connectors_changed && !new_crtc_state->enable)
> > +		to_intel_crtc_state(new_crtc_state)->dp_m_n.tu = 0;
> 
> No idea why we need to clear this here? Shouldn't we just clear this
> unconditionally if we release the allocation.
> 
> Also naming convention, I'd call this old_crtc_new_state, and maybe
> old_crtc_old_state. To make it clear these are the old/new states for
> old_crtc. Instead of old/new state for the new crtc.
> 
> >  
> > -		ret = drm_dp_atomic_release_vcpi_slots(state, mgr, slots);
> > -		if (ret)
> > -			DRM_DEBUG_KMS("failed releasing %d vcpi slots:%d\n",
> > slots, ret);
> > -		else
> > -			to_intel_crtc_state(crtc_state)->dp_m_n.tu = 0;
> > -	}
> > -	return ret;
> > +	if (new_crtc)
> > +		return 0;
> 
> Not sure why we shouldn't release this? If we handle this in helpers, it
> would remove piles of special casing code here I think. Plus remove
> special code in the hlpers.
> 
> 
> > +
> > +	return drm_dp_atomic_release_vcpi_slots(state, mgr, port);
> >  }
> >  
> >  static void intel_mst_disable_dp(struct intel_encoder *encoder,
> > diff --git a/include/drm/drm_dp_mst_helper.h
> > b/include/drm/drm_dp_mst_helper.h
> > index 3faceb66f5cb..2392e12b6bce 100644
> > --- a/include/drm/drm_dp_mst_helper.h
> > +++ b/include/drm/drm_dp_mst_helper.h
> > @@ -406,9 +406,16 @@ struct drm_dp_payload {
> >  
> >  #define to_dp_mst_topology_state(x) container_of(x, struct
> > drm_dp_mst_topology_state, base)
> >  
> > +struct drm_dp_vcpi_allocation {
> 
> I think a comment here that this pointer holds a full reference (even past
> when the connector is unplugged) would be good. Otherwise the loop in
> release_vcpi would be buggy.
> 
> > +	struct drm_dp_mst_port *port;
> > +	int vcpi;
> > +	struct list_head next;
> > +};
> > +
> >  struct drm_dp_mst_topology_state {
> >  	struct drm_private_state base;
> > -	int avail_slots;
> > +	struct list_head vcpis;
> > +	u8 vcpi_allocated;
> >  	struct drm_dp_mst_topology_mgr *mgr;
> >  };
> >  
> > @@ -619,14 +626,17 @@ void drm_dp_mst_topology_mgr_suspend(struct
> > drm_dp_mst_topology_mgr *mgr);
> >  int drm_dp_mst_topology_mgr_resume(struct drm_dp_mst_topology_mgr *mgr);
> >  struct drm_dp_mst_topology_state
> > *drm_atomic_get_mst_topology_state(struct drm_atomic_state *state,
> >  								    struct
> > drm_dp_mst_topology_mgr *mgr);
> > -int drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state *state,
> > -				  struct drm_dp_mst_topology_mgr *mgr,
> > -				  struct drm_dp_mst_port *port, int pbn);
> > -int drm_dp_atomic_release_vcpi_slots(struct drm_atomic_state *state,
> > -				     struct drm_dp_mst_topology_mgr *mgr,
> > -				     int slots);
> > +int __must_check
> > +drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state *state,
> > +			      struct drm_dp_mst_topology_mgr *mgr,
> > +			      struct drm_dp_mst_port *port, int pbn);
> > +int __must_check
> > +drm_dp_atomic_release_vcpi_slots(struct drm_atomic_state *state,
> > +				 struct drm_dp_mst_topology_mgr *mgr,
> > +				 struct drm_dp_mst_port *port);
> >  int drm_dp_send_power_updown_phy(struct drm_dp_mst_topology_mgr *mgr,
> >  				 struct drm_dp_mst_port *port, bool power_up);
> > +int __must_check drm_dp_mst_atomic_check(struct drm_atomic_state *state);
> >  
> >  extern const struct drm_private_state_funcs
> > drm_dp_mst_topology_state_funcs;
> >  
> > -- 
> > 2.19.1
> >

Daniel Vetter Nov. 27, 2018, 9:07 a.m. UTC | #4

On Mon, Nov 26, 2018 at 04:36:46PM -0500, Lyude Paul wrote:
> On Mon, 2018-11-26 at 22:04 +0100, Daniel Vetter wrote:
> > On Thu, Nov 15, 2018 at 07:50:05PM -0500, Lyude Paul wrote:
> > > There has been a TODO waiting for quite a long time in
> > > drm_dp_mst_topology.c:
> > > 
> > > 	/* We cannot rely on port->vcpi.num_slots to update
> > > 	 * topology_state->avail_slots as the port may not exist if the parent
> > > 	 * branch device was unplugged. This should be fixed by tracking
> > > 	 * per-port slot allocation in drm_dp_mst_topology_state instead of
> > > 	 * depending on the caller to tell us how many slots to release.
> > > 	 */
> > > 
> > > That's not the only reason we should fix this: forcing the driver to
> > > track the VCPI allocations throughout a state's atomic check is
> > > error prone, because it means that extra care has to be taken with the
> > > order that drm_dp_atomic_find_vcpi_slots() and
> > > drm_dp_atomic_release_vcpi_slots() are called in in order to ensure
> > > idempotency. Currently the only driver actually using these helpers,
> > > i915, doesn't even do this correctly: multiple ->best_encoder() checks
> > > with i915's current implementation would not be idempotent and would
> > > over-allocate VCPI slots, something I learned trying to implement
> > > fallback retraining in MST.
> > > 
> > > So: simplify this whole mess, and teach drm_dp_atomic_find_vcpi_slots()
> > > and drm_dp_atomic_release_vcpi_slots() to track the VCPI allocations for
> > > each port. This allows us to ensure idempotency without having to rely
> > > on the driver as much. Additionally: the driver doesn't need to do any
> > > kind of VCPI slot tracking anymore if it doesn't need it for it's own
> > > internal state.
> > > 
> > > Additionally; this adds a new drm_dp_mst_atomic_check() helper which
> > > must be used by atomic drivers to perform validity checks for the new
> > > VCPI allocations incurred by a state.
> > > 
> > > Also: update the documentation and make it more obvious that these
> > > /must/ be called by /all/ atomic drivers supporting MST.
> > > 
> > > Changes since v6:
> > >  - Keep a kref to all of the ports we have allocations on. This required
> > >    a good bit of changing to when we call drm_dp_find_vcpi_slots(),
> > >    mainly that we need to ensure that we only redo VCPI allocations on
> > >    actual mode or CRTC changes, not crtc_state->active changes.
> > >    Additionally, we no longer take the registration of the DRM connector
> > >    for each port into account because so long as we have a kref to the
> > >    port in the new or previous atomic state, the connector will stay
> > >    registered.
> > 
> > I write an entire pile of small nitpits (still included most of them
> > below), until I realized this here wont work. Delaying the call to destroy
> > the connector (well, unregister it really) wreaks the design we've come up
> > with thus far, resulting in most of my comments here.
> > 
> > Instead, all we need to do is delay the kfree(port) at the bottom of
> > drm_dp_destroy_port(). The vcpi allocation structure _only_ needs the
> > pointer value to stay valid, as a lookup key. It doesn't care at all about
> > anything actually stored in there. So the only thing we need to delay is
> > the kfree. I think the simplest way to achieve this is to add a 2nd kref
> > (port->kfree_ref or something like that), with on reference held by the
> > port itself (released in drm_dp_destroy_port), and the other one held
> > as-needed by the vcpi allocation lists.
> > 
> > I think if we go with this design instead of retrofitting a semantic
> > change of the port lifetime itself, all the complications I complain about
> > below should disappear.
> > 
> > Piles of comments below.
> > 
> > Cheers, Daniel
> > 
> > >  - Use the small changes to drm_dp_put_port() to add even more error
> > >    checking to make misusage of the helpers more obvious. I added this
> > >    after having to chase down various use-after-free conditions that
> > >    started popping up from the new helpers so no one else has to
> > >    troubleshoot that.
> > >  - Move some accidental DRM_DEBUG_KMS() calls to DRM_DEBUG_ATOMIC()
> > >  - Update documentation again, note that find/release() should both not be
> > >    called on the same port in a single atomic check phase (but multiple
> > >    calls to one or the other is OK)
> > > 
> > > Changes since v4:
> > >  - Don't skip the atomic checks for VCPI allocations if no new VCPI
> > >    allocations happen in a state. This makes the next change I'm about
> > >    to list here a lot easier to implement.
> > >  - Don't ignore VCPI allocations on destroyed ports, instead ensure that
> > >    when ports are destroyed and still have VCPI allocations in the
> > >    topology state, the only state changes allowed are releasing said
> > >    ports' VCPI. This prevents a state with a mix of VCPI allocations
> > >    from destroyed ports, and allocations from valid ports.
> > > 
> > > Changes since v3:
> > >  - Don't release VCPI allocations in the topology state immediately in
> > >    drm_dp_atomic_release_vcpi_slots(), instead mark them as 0 and skip
> > >    over them in drm_dp_mst_duplicate_state(). This makes it so
> > >    drm_dp_atomic_release_vcpi_slots() is still idempotent while also
> > >    throwing warnings if the driver messes up it's book keeping and tries
> > >    to release VCPI slots on a port that doesn't have any pre-existing
> > >    VCPI allocation - danvet
> > >  - Change mst_state/state in some debugging messages to "mst state"
> > > 
> > > Changes since v2:
> > >  - Use kmemdup() for duplicating MST state - danvet
> > >  - Move port validation out of duplicate state callback - danvet
> > >  - Handle looping through MST topology states in
> > >    drm_dp_mst_atomic_check() so the driver doesn't have to do it
> > >  - Fix documentation in drm_dp_atomic_find_vcpi_slots()
> > >  - Move the atomic check for each individual topology state into it's
> > >    own function, reduces indenting
> > >  - Don't consider "stale" MST ports when calculating the bandwidth
> > >    requirements. This is needed because originally we relied on the
> > >    state duplication functions to prune any stale ports from the new
> > >    state, which would prevent us from incorrectly considering their
> > >    bandwidth requirements alongside legitimate new payloads.
> > >  - Add function references in drm_dp_atomic_release_vcpi_slots() - danvet
> > >  - Annotate atomic VCPI and atomic check functions with __must_check
> > >    - danvet
> > > 
> > > Changes since v1:
> > >  - Don't use the now-removed ->atomic_check() for private objects hook,
> > >    just give drivers a function to call themselves
> > > 
> > > Signed-off-by: Lyude Paul <[e-mail]>
> > > Cc: Daniel Vetter <[e-mail]>
> > > ---
> > >  drivers/gpu/drm/drm_dp_mst_topology.c | 294 +++++++++++++++++++++++---
> > >  drivers/gpu/drm/i915/intel_display.c  |   4 +
> > >  drivers/gpu/drm/i915/intel_dp_mst.c   |  64 +++---
> > >  include/drm/drm_dp_mst_helper.h       |  24 ++-
> > >  4 files changed, 319 insertions(+), 67 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c
> > > b/drivers/gpu/drm/drm_dp_mst_topology.c
> > > index 00fbe7a2699d..183d0e832ced 100644
> > > --- a/drivers/gpu/drm/drm_dp_mst_topology.c
> > > +++ b/drivers/gpu/drm/drm_dp_mst_topology.c
> > > @@ -2617,21 +2617,42 @@ static int drm_dp_init_vcpi(struct
> > > drm_dp_mst_topology_mgr *mgr,
> > >  }
> > >  
> > >  /**
> > > - * drm_dp_atomic_find_vcpi_slots() - Find and add vcpi slots to the state
> > > + * drm_dp_atomic_find_vcpi_slots() - Find and add VCPI slots to the state
> > >   * @state: global atomic state
> > >   * @mgr: MST topology manager for the port
> > >   * @port: port to find vcpi slots for
> > >   * @pbn: bandwidth required for the mode in PBN
> > >   *
> > > - * RETURNS:
> > > - * Total slots in the atomic state assigned for this port or error
> > > + * Allocates VCPI slots to @port, replacing any previous VCPI allocations
> > > it
> > > + * may have had. Any atomic drivers which support MST must call this
> > > function
> > > + * in their &drm_encoder_helper_funcs.atomic_check() callback to change
> > > the
> > > + * current VCPI allocation for the new state, but only when
> > > + * &drm_crtc_state.mode_changed or &drm_crtc_state.connectors_changed is
> > > set
> > > + * to ensure compatibility with userspace applications that still use the
> > > + * legacy modesetting UAPI.
> > > + *
> > > + * Allocations set by this function are not checked against the bandwidth
> > > + * restraints of @mgr until the driver calls drm_dp_mst_atomic_check().
> > > + *
> > > + * Additionally, it is OK to call this function multiple times on the
> > > same
> > > + * @port as needed. It is not OK however, to call this function and
> > > + * drm_dp_atomic_release_vcpi_slots() in the same atomic check phase.
> > > + *
> > > + * See also:
> > > + * drm_dp_atomic_release_vcpi_slots()
> > > + * drm_dp_mst_atomic_check()
> > > + *
> > > + * Returns:
> > > + * Total slots in the atomic state assigned for this port, or a negative
> > > error
> > > + * code if the port no longer exists
> > >   */
> > >  int drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state *state,
> > >  				  struct drm_dp_mst_topology_mgr *mgr,
> > >  				  struct drm_dp_mst_port *port, int pbn)
> > >  {
> > >  	struct drm_dp_mst_topology_state *topology_state;
> > > -	int req_slots;
> > > +	struct drm_dp_vcpi_allocation *pos, *vcpi = NULL;
> > > +	int prev_slots, req_slots, ret;
> > >  
> > >  	topology_state = drm_atomic_get_mst_topology_state(state, mgr);
> > >  	if (IS_ERR(topology_state))
> > > @@ -2640,20 +2661,60 @@ int drm_dp_atomic_find_vcpi_slots(struct
> > > drm_atomic_state *state,
> > >  	port = drm_dp_get_validated_port_ref(mgr, port);
> > >  	if (port == NULL)
> > >  		return -EINVAL;
> > > -	req_slots = DIV_ROUND_UP(pbn, mgr->pbn_div);
> > > -	DRM_DEBUG_KMS("vcpi slots req=%d, avail=%d\n",
> > > -			req_slots, topology_state->avail_slots);
> > >  
> > > -	if (req_slots > topology_state->avail_slots) {
> > > -		drm_dp_put_port(port);
> > > -		return -ENOSPC;
> > > +	topology_state->vcpi_allocated = true;
> > > +
> > > +	/* Find the current allocation for this port, if any */
> > > +	list_for_each_entry(pos, &topology_state->vcpis, next) {
> > > +		if (pos->port == port) {
> > > +			vcpi = pos;
> > > +			prev_slots = vcpi->vcpi;
> > > +
> > > +			/*
> > > +			 * This should never happen, unless the driver tries
> > > +			 * releasing and allocating the same VCPI allocation,
> > > +			 * which is an error
> > > +			 */
> > > +			if (WARN_ON(!prev_slots)) {
> > > +				DRM_ERROR("cannot allocate and release VCPI on
> > > [MST PORT:%p] in the same state\n",
> > > +					  port);
> > > +				return -EINVAL;
> > > +			}
> > 
> > Allowing atomic changes to output state (like disabling it on one crtc,
> > and then enabling on the other, thus both releasing and re-allocating the
> > vcpi in one go) is kinda the point of atomic. So definitely can't EINVAL
> > here.
> Actually-so long the original assertion we had of not allowing "stale" (e.g.
> VCPIs from now-destroyed topologies) VCPI allocations and real VCPI
> allocations in the same atomic state is held true, this should be fine.
> 
> So: (and this answers the question below as well), the way this is supposed to
> work is that we just skip the whole VCPI validation sequence unless part of
> the state actually tried to add VCPI. This means userland can still disable
> displays one-by-one or set them as inactive, so long as it doesn't try to
> enable new MST displays until the point at which it's disabled everything
> leftover from the previous topology - since disabling displays won't allocate
> VCPI and thus skips the port validation checks here.

This doesn't sound robust if there's a race, e.g. userspace tries to light
up a display right at the time when we unplug a different (already enabled
output). I think to fix this properly we need to replace all the
dp_get_validated_port calls in all the vcpi/payload places with the
lower-level kref that only prevents the kfree.

Fundamentally hotunplug is still racy, but this way I think there's at
least no surprises. And we could probably remove a bunch of the hacks
around cleanup paths.

> > Sounds like we need more testcases in igt?
> > 
> > > +
> > > +			break;
> > > +		}
> > >  	}
> > > +	if (!vcpi)
> > > +		prev_slots = 0;
> > >  
> > > -	topology_state->avail_slots -= req_slots;
> > > -	DRM_DEBUG_KMS("vcpi slots avail=%d", topology_state->avail_slots);
> > > +	req_slots = DIV_ROUND_UP(pbn, mgr->pbn_div);
> > > +
> > > +	DRM_DEBUG_ATOMIC("[CONNECTOR:%d:%s] [MST PORT:%p] VCPI %d -> %d\n",
> > > +			 port->connector->base.id, port->connector->name,
> > > +			 port, prev_slots, req_slots);
> > > +
> > > +	/* Add the new allocation to the state */
> > > +	if (!vcpi) {
> > > +		vcpi = kzalloc(sizeof(*vcpi), GFP_KERNEL);
> > > +		if (!vcpi) {
> > > +			ret = -ENOMEM;
> > > +			goto out;
> > > +		}
> > >  
> > > +		vcpi->port = drm_dp_get_validated_port_ref(mgr, port);
> > > +		if (!vcpi->port) {
> > > +			ret = -EINVAL;
> > > +			kfree(vcpi);
> > > +			goto out;
> > > +		}
> > > +		list_add(&vcpi->next, &topology_state->vcpis);
> > > +	}
> > > +	vcpi->vcpi = req_slots;
> > > +
> > > +	ret = req_slots;
> > > +out:
> > >  	drm_dp_put_port(port);
> > > -	return req_slots;
> > > +	return ret;
> > >  }
> > >  EXPORT_SYMBOL(drm_dp_atomic_find_vcpi_slots);
> > >  
> > > @@ -2661,31 +2722,66 @@ EXPORT_SYMBOL(drm_dp_atomic_find_vcpi_slots);
> > >   * drm_dp_atomic_release_vcpi_slots() - Release allocated vcpi slots
> > >   * @state: global atomic state
> > >   * @mgr: MST topology manager for the port
> > > - * @slots: number of vcpi slots to release
> > > + * @port: The port to release the VCPI slots from
> > >   *
> > > - * RETURNS:
> > > - * 0 if @slots were added back to &drm_dp_mst_topology_state->avail_slots 
> > > or
> > > - * negative error code
> > > + * Releases any VCPI slots that have been allocated to a port in the
> > > atomic
> > > + * state. Any atomic drivers which support MST must call this function in
> > > + * their &drm_connector_helper_funcs.atomic_check() callback when the
> > > + * connector will no longer have VCPI allocated (e.g. because it's CRTC
> > > was
> > > + * removed) when it had VCPI allocated in the previous atomic state.
> > > + *
> > > + * It is OK to call this even if @port has been removed from the system.
> > > + * Additionally, it is OK to call this function multiple times on the
> > > same
> > > + * @port as needed. It is not OK however, to call this function and
> > > + * drm_dp_atomic_find_vcpi_slots() on the same @port in a single atomic
> > > check
> > > + * phase.
> > > + *
> > > + * See also:
> > > + * drm_dp_atomic_find_vcpi_slots()
> > > + * drm_dp_mst_atomic_check()
> > > + *
> > > + * Returns:
> > > + * 0 if all slots for this port were added back to
> > > + * &drm_dp_mst_topology_state.avail_slots or negative error code
> > >   */
> > >  int drm_dp_atomic_release_vcpi_slots(struct drm_atomic_state *state,
> > >  				     struct drm_dp_mst_topology_mgr *mgr,
> > > -				     int slots)
> > > +				     struct drm_dp_mst_port *port)
> > >  {
> > >  	struct drm_dp_mst_topology_state *topology_state;
> > > +	struct drm_dp_vcpi_allocation *pos;
> > > +	bool found = false;
> > >  
> > >  	topology_state = drm_atomic_get_mst_topology_state(state, mgr);
> > >  	if (IS_ERR(topology_state))
> > >  		return PTR_ERR(topology_state);
> > >  
> > > -	/* We cannot rely on port->vcpi.num_slots to update
> > > -	 * topology_state->avail_slots as the port may not exist if the parent
> > > -	 * branch device was unplugged. This should be fixed by tracking
> > > -	 * per-port slot allocation in drm_dp_mst_topology_state instead of
> > > -	 * depending on the caller to tell us how many slots to release.
> > > -	 */
> > > -	topology_state->avail_slots += slots;
> > > -	DRM_DEBUG_KMS("vcpi slots released=%d, avail=%d\n",
> > > -			slots, topology_state->avail_slots);
> > > +	list_for_each_entry(pos, &topology_state->vcpis, next) {
> > > +		if (pos->port == port) {
> > > +			found = true;
> > > +			break;
> > > +		}
> > > +	}
> > > +	if (WARN_ON(!found)) {
> > > +		DRM_ERROR("no VCPI for [MST PORT:%p] found in mst state %p\n",
> > > +			  port, &topology_state->base);
> > > +		return -EINVAL;
> > > +	}
> > > +
> > > +	DRM_DEBUG_ATOMIC("[MST PORT:%p] VCPI %d -> 0\n", port, pos->vcpi);
> > > +	if (pos->vcpi) {
> > > +		/*
> > > +		 * This should be safe-as the previous state will still hold a
> > > +		 * reference to this port until we swap states. If it's not,
> > > +		 * the driver has a bookkeeping error
> > > +		 */
> > > +		if (WARN_ON(drm_dp_put_port(port))) {
> > 
> > The usual way to do this (instead of adding a return value to the normal
> > _put function) is something like
> > 
> > 	kref_put(port, function_that_with_just_a_WARNING_DRM_ERROR)
> > 
> > avoids the need for patch 2.
> > 
> > > +			DRM_ERROR("cannot allocate and release VCPI on [MST
> > > PORT:%p] in the same state\n",
> > > +				  port);
> > > +			return -EINVAL;
> > > +		}
> > > +		pos->vcpi = 0;
> > > +	}
> > >  
> > >  	return 0;
> > >  }
> > > @@ -3115,15 +3211,45 @@ static void drm_dp_destroy_connector_work(struct
> > > work_struct *work)
> > >  static struct drm_private_state *
> > >  drm_dp_mst_duplicate_state(struct drm_private_obj *obj)
> > >  {
> > > -	struct drm_dp_mst_topology_state *state;
> > > +	struct drm_dp_mst_topology_state *state, *old_state =
> > > +		to_dp_mst_topology_state(obj->state);
> > > +	struct drm_dp_vcpi_allocation *pos, *vcpi;
> > >  
> > > -	state = kmemdup(obj->state, sizeof(*state), GFP_KERNEL);
> > > +	state = kmemdup(old_state, sizeof(*state), GFP_KERNEL);
> > >  	if (!state)
> > >  		return NULL;
> > >  
> > >  	__drm_atomic_helper_private_obj_duplicate_state(obj, &state->base);
> > >  
> > > +	INIT_LIST_HEAD(&state->vcpis);
> > > +	state->vcpi_allocated = false;
> > > +
> > > +	list_for_each_entry(pos, &old_state->vcpis, next) {
> > > +		/* Prune leftover freed VCPI allocations */
> > > +		if (!pos->vcpi)
> > > +			continue;
> > > +
> > > +		vcpi = kmemdup(pos, sizeof(*vcpi), GFP_KERNEL);
> > > +		if (!vcpi)
> > > +			goto fail;
> > > +
> > > +		/* Take a plain old kref here and don't validate the port, as
> > > +		 * it's topology might not even exist anymore
> > > +		 */
> > > +		kref_get(&vcpi->port->kref);
> > > +		list_add(&vcpi->next, &state->vcpis);
> > > +	}
> > > +
> > >  	return &state->base;
> > > +
> > > +fail:
> > > +	list_for_each_entry_safe(pos, vcpi, &state->vcpis, next) {
> > > +		drm_dp_put_port(pos->port);
> > > +		kfree(pos);
> > > +	}
> > > +	kfree(state);
> > > +
> > > +	return NULL;
> > >  }
> > >  
> > >  static void drm_dp_mst_destroy_state(struct drm_private_obj *obj,
> > > @@ -3131,14 +3257,116 @@ static void drm_dp_mst_destroy_state(struct
> > > drm_private_obj *obj,
> > >  {
> > >  	struct drm_dp_mst_topology_state *mst_state =
> > >  		to_dp_mst_topology_state(state);
> > > +	struct drm_dp_vcpi_allocation *pos, *tmp;
> > > +
> > > +	list_for_each_entry_safe(pos, tmp, &mst_state->vcpis, next) {
> > > +		/* We only keep references to ports with non-zero VCPIs */
> > > +		if (pos->vcpi)
> > > +			drm_dp_put_port(pos->port);
> > > +		kfree(pos);
> > > +	}
> > >  
> > >  	kfree(mst_state);
> > >  }
> > >  
> > > -static const struct drm_private_state_funcs mst_state_funcs = {
> > > +static inline int
> > > +drm_dp_mst_atomic_check_topology_state(struct drm_dp_mst_topology_mgr
> > > *mgr,
> > > +				       struct drm_dp_mst_topology_state
> > > *mst_state)
> > > +{
> > > +	struct drm_dp_vcpi_allocation *vcpi;
> > > +	struct drm_dp_mst_port *port;
> > > +	int avail_slots = 63, ret;
> > > +
> > > +	/* There's no possible scenario where releasing VCPI or keeping it the
> > > +	 * same would make the state invalid
> > > +	 */
> > > +	if (!mst_state->vcpi_allocated) {
> > 
> > Do we need this? If not I'm always in favour of removing code if it's not
> > needed :-)
> answered by my explanation above ^

If removing this breaks the code then I think it's a bit fragile. And see
above, open to races (yes unlikely ones, but then all of mst is full of
unlikely races it that still seem to happen eventually).

I think it's better if we can remove all these special cases (which should
be possible if we drop the get_validated_port below).

> > 
> > > +		DRM_DEBUG_ATOMIC("[MST MGR:%p] state %p allocates no VCPI,
> > > check skipped\n",
> > > +				 mgr, &mst_state->base);
> > > +		return 0;
> > > +	}
> > > +
> > > +	list_for_each_entry(vcpi, &mst_state->vcpis, next) {
> > > +		/* Releasing VCPI is always OK-even if the port is gone */
> > > +		if (!vcpi->vcpi) {
> > > +			DRM_DEBUG_ATOMIC("[MST PORT:%p] releases all VCPI
> > > slots\n",
> > > +					 vcpi->port);
> > > +			continue;
> > > +		}
> > > +
> > > +		port = drm_dp_get_validated_port_ref(mgr, vcpi->port);
> > > +		if (!port) {
> > > +			DRM_DEBUG_ATOMIC("[MST PORT:%p] is gone but still has
> > > VCPI, cannot add new VCPI\n",
> > > +					 vcpi->port);
> > > +			return -EINVAL;
> > > +		}
> > 
> > For find_vcpi we've already done this. For unplugged vcpi, we can't do
> > this here I think - otherwise if you enable 2 mst outputs on the same
> > branch, and unplug both, old non-atomic userspace can never ever again
> > disable them because it can't disable both. The check (to skip already
> > deallocated ports) above isn't enough to handle that.
> 
> answered by my explanation above ^
> > 
> > > +
> > > +		DRM_DEBUG_ATOMIC("[MST PORT:%p] requires %d vcpi slots\n",
> > > +				 vcpi->port, vcpi->vcpi);
> > > +
> > > +		avail_slots -= vcpi->vcpi;
> > > +		if (avail_slots < 0) {
> > > +			DRM_DEBUG_ATOMIC("[MST PORT:%p] not enough VCPI slots
> > > in mst state %p (avail=%d)\n",
> > > +					 vcpi->port, mst_state,
> > > +					 avail_slots + vcpi->vcpi);
> > > +			ret = -ENOSPC;
> > > +			goto port_fail;
> > > +		}
> > > +
> > > +		drm_dp_put_port(port);
> > > +	}
> > > +	DRM_DEBUG_ATOMIC("[MST MGR:%p] mst state %p VCPI avail=%d used=%d\n",
> > > +			 mgr, mst_state, avail_slots,
> > > +			 63 - avail_slots);
> > > +
> > > +	return 0;
> > > +port_fail:
> > > +	drm_dp_put_port(port);
> > > +	return ret;
> > > +}
> > > +
> > > +/**
> > > + * drm_dp_mst_atomic_check - Check that the new state of an MST topology
> > > in an
> > > + * atomic update is valid
> > > + * @state: Pointer to the new &struct drm_dp_mst_topology_state
> > > + *
> > > + * Checks the given topology state for an atomic update to ensure that
> > > it's
> > > + * valid. This includes checking whether there's enough bandwidth to
> > > support
> > > + * the new VCPI allocations in the atomic update.
> > > + *
> > > + * Any atomic drivers supporting DP MST must make sure to call this after
> > > + * checking the rest of their state in their
> > > + * &drm_mode_config_funcs.atomic_check() callback.
> > > + *
> > > + * See also:
> > > + * drm_dp_atomic_find_vcpi_slots()
> > > + * drm_dp_atomic_release_vcpi_slots()
> > > + *
> > > + * Returns:
> > > + *
> > > + * 0 if the new state is valid, negative error code otherwise.
> > > + */
> > > +int drm_dp_mst_atomic_check(struct drm_atomic_state *state)
> > > +{
> > > +	struct drm_dp_mst_topology_mgr *mgr;
> > > +	struct drm_dp_mst_topology_state *mst_state;
> > > +	int i, ret = 0;
> > > +
> > > +	for_each_new_mst_mgr_in_state(state, mgr, mst_state, i) {
> > > +		ret = drm_dp_mst_atomic_check_topology_state(mgr, mst_state);
> > > +		if (ret)
> > > +			break;
> > > +	}
> > > +
> > > +	return ret;
> > > +}
> > > +EXPORT_SYMBOL(drm_dp_mst_atomic_check);
> > > +
> > > +const struct drm_private_state_funcs drm_dp_mst_topology_state_funcs = {
> > >  	.atomic_duplicate_state = drm_dp_mst_duplicate_state,
> > >  	.atomic_destroy_state = drm_dp_mst_destroy_state,
> > >  };
> > > +EXPORT_SYMBOL(drm_dp_mst_topology_state_funcs);
> > >  
> > >  /**
> > >   * drm_atomic_get_mst_topology_state: get MST topology state
> > > @@ -3216,13 +3444,11 @@ int drm_dp_mst_topology_mgr_init(struct
> > > drm_dp_mst_topology_mgr *mgr,
> > >  		return -ENOMEM;
> > >  
> > >  	mst_state->mgr = mgr;
> > > -
> > > -	/* max. time slots - one slot for MTP header */
> > > -	mst_state->avail_slots = 63;
> > > +	INIT_LIST_HEAD(&mst_state->vcpis);
> > >  
> > >  	drm_atomic_private_obj_init(&mgr->base,
> > >  				    &mst_state->base,
> > > -				    &mst_state_funcs);
> > > +				    &drm_dp_mst_topology_state_funcs);
> > >  
> > >  	return 0;
> > >  }
> > > diff --git a/drivers/gpu/drm/i915/intel_display.c
> > > b/drivers/gpu/drm/i915/intel_display.c
> > > index 132e978227fb..8e85e04a5948 100644
> > > --- a/drivers/gpu/drm/i915/intel_display.c
> > > +++ b/drivers/gpu/drm/i915/intel_display.c
> > > @@ -12592,6 +12592,10 @@ static int intel_atomic_check(struct drm_device
> > > *dev,
> > >  				       "[modeset]" : "[fastset]");
> > >  	}
> > >  
> > > +	ret = drm_dp_mst_atomic_check(state);
> > > +	if (ret)
> > > +		return ret;
> > > +
> > >  	if (any_ms) {
> > >  		ret = intel_modeset_checks(state);
> > >  
> > > diff --git a/drivers/gpu/drm/i915/intel_dp_mst.c
> > > b/drivers/gpu/drm/i915/intel_dp_mst.c
> > > index 4de247ddf05f..be2d1ac7d87d 100644
> > > --- a/drivers/gpu/drm/i915/intel_dp_mst.c
> > > +++ b/drivers/gpu/drm/i915/intel_dp_mst.c
> > > @@ -41,8 +41,13 @@ static bool intel_dp_mst_compute_config(struct
> > > intel_encoder *encoder,
> > >  	struct drm_connector *connector = conn_state->connector;
> > >  	void *port = to_intel_connector(connector)->port;
> > >  	struct drm_atomic_state *state = pipe_config->base.state;
> > > +	struct drm_crtc *crtc = pipe_config->base.crtc;
> > > +	struct drm_crtc_state *old_crtc_state =
> > > +		drm_atomic_get_old_crtc_state(state, crtc);
> > > +	struct drm_crtc_state *new_crtc_state = &pipe_config->base;
> > >  	int bpp;
> > > -	int lane_count, slots = 0;
> > > +	int lane_count, slots =
> > > +		to_intel_crtc_state(old_crtc_state)->dp_m_n.tu;
> > >  	const struct drm_display_mode *adjusted_mode = &pipe_config-
> > > >base.adjusted_mode;
> > >  	int mst_pbn;
> > >  	bool constant_n = drm_dp_has_quirk(&intel_dp->desc,
> > > @@ -77,8 +82,12 @@ static bool intel_dp_mst_compute_config(struct
> > > intel_encoder *encoder,
> > >  	mst_pbn = drm_dp_calc_pbn_mode(adjusted_mode->crtc_clock, bpp);
> > >  	pipe_config->pbn = mst_pbn;
> > >  
> > > -	/* Zombie connectors can't have VCPI slots */
> > > -	if (!drm_connector_is_unregistered(connector)) {
> > > +	/* Only change VCPI allocation on actual mode changes, to prevent us
> > > +	 * from trying to allocate VCPI to ports that no longer exist when we
> > > +	 * may just be trying to disable DPMS on them
> > > +	 */
> > > +	if (new_crtc_state->mode_changed ||
> > > +	    new_crtc_state->connectors_changed) {
> > 
> > Why exactly is this needed? We only call compute_config on the enocoder
> > when enabling an output. Not when disabling one. Similar for the atomic
> > helpers and the various encoder callbacks (you have the same change in the
> > nouveau code). Did this actually blow up while testing?
> 
> yes-it looked like we were calling compute_config on simple DPMS on->off
> chnges as well, this ended up making it so we wouldn't try changing the VCPI
> allocation on such commits.

Ah right, we discussed all this when adding the is_unregistered() check.
If we don't change the port lifetime (but only when kfree is called) then
there's no need to change anything here. Allows us to keep the nice
consistency of the is_unregistered() check in driver code.

I should have rewritten this when I realized that you've changed the
lifetime rules in a different way than what I had in mind, as-is my
comment didn't make much sense.
-Daniel

> ACK on all of the other comments here.
> > 
> > >  		slots = drm_dp_atomic_find_vcpi_slots(state,
> > >  						      &intel_dp->mst_mgr,
> > >  						      port,
> > > @@ -107,36 +116,39 @@ static bool intel_dp_mst_compute_config(struct
> > > intel_encoder *encoder,
> > >  	return true;
> > >  }
> > >  
> > > -static int intel_dp_mst_atomic_check(struct drm_connector *connector,
> > > -		struct drm_connector_state *new_conn_state)
> > > +static int
> > > +intel_dp_mst_atomic_check(struct drm_connector *connector,
> > > +			  struct drm_connector_state *new_conn_state)
> > >  {
> > > +	struct intel_connector *intel_connector =
> > > +		to_intel_connector(connector);
> > > +	struct drm_dp_mst_topology_mgr *mgr =
> > > +		&intel_connector->mst_port->mst_mgr;
> > > +	struct drm_dp_mst_port *port = intel_connector->port;
> > >  	struct drm_atomic_state *state = new_conn_state->state;
> > > -	struct drm_connector_state *old_conn_state;
> > > -	struct drm_crtc *old_crtc;
> > > -	struct drm_crtc_state *crtc_state;
> > > -	int slots, ret = 0;
> > > +	struct drm_connector_state *old_conn_state =
> > > +		drm_atomic_get_old_connector_state(state, connector);
> > > +	struct drm_crtc *old_crtc = old_conn_state->crtc,
> > > +			*new_crtc = new_conn_state->crtc;
> > > +	struct drm_crtc_state *old_crtc_state, *new_crtc_state;
> > >  
> > > -	old_conn_state = drm_atomic_get_old_connector_state(state, connector);
> > > -	old_crtc = old_conn_state->crtc;
> > 
> > Bikeshed: Reworking all the above hides the real changes quite a bit.
> > 
> > >  	if (!old_crtc)
> > > -		return ret;
> > > +		return 0;
> > >  
> > > -	crtc_state = drm_atomic_get_new_crtc_state(state, old_crtc);
> > > -	slots = to_intel_crtc_state(crtc_state)->dp_m_n.tu;
> > > -	if (drm_atomic_crtc_needs_modeset(crtc_state) && slots > 0) {
> > > -		struct drm_dp_mst_topology_mgr *mgr;
> > > -		struct drm_encoder *old_encoder;
> > > +	old_crtc_state = drm_atomic_get_old_crtc_state(state, old_crtc);
> > > +	if (!old_crtc_state ||
> > 
> > !old_crtc_state would be a framework bug. Just remove that check.
> > 
> > > +	    !to_intel_crtc_state(old_crtc_state)->dp_m_n.tu)
> > > +		return 0;
> > 
> > Hm, why do we need this? Having a DP port previously enabled with a 0 tu
> > sounds like a pretty big driver bug.
> > >  
> > > -		old_encoder = old_conn_state->best_encoder;
> > > -		mgr = &enc_to_mst(old_encoder)->primary->dp.mst_mgr;
> > > +	/* If we switched CRTCs, clear the previous one's allocation */
> > > +	new_crtc_state = drm_atomic_get_new_crtc_state(state, old_crtc);
> > > +	if (new_crtc_state->connectors_changed && !new_crtc_state->enable)
> > > +		to_intel_crtc_state(new_crtc_state)->dp_m_n.tu = 0;
> > 
> > No idea why we need to clear this here? Shouldn't we just clear this
> > unconditionally if we release the allocation.
> > 
> > Also naming convention, I'd call this old_crtc_new_state, and maybe
> > old_crtc_old_state. To make it clear these are the old/new states for
> > old_crtc. Instead of old/new state for the new crtc.
> > 
> > >  
> > > -		ret = drm_dp_atomic_release_vcpi_slots(state, mgr, slots);
> > > -		if (ret)
> > > -			DRM_DEBUG_KMS("failed releasing %d vcpi slots:%d\n",
> > > slots, ret);
> > > -		else
> > > -			to_intel_crtc_state(crtc_state)->dp_m_n.tu = 0;
> > > -	}
> > > -	return ret;
> > > +	if (new_crtc)
> > > +		return 0;
> > 
> > Not sure why we shouldn't release this? If we handle this in helpers, it
> > would remove piles of special casing code here I think. Plus remove
> > special code in the hlpers.
> > 
> > 
> > > +
> > > +	return drm_dp_atomic_release_vcpi_slots(state, mgr, port);
> > >  }
> > >  
> > >  static void intel_mst_disable_dp(struct intel_encoder *encoder,
> > > diff --git a/include/drm/drm_dp_mst_helper.h
> > > b/include/drm/drm_dp_mst_helper.h
> > > index 3faceb66f5cb..2392e12b6bce 100644
> > > --- a/include/drm/drm_dp_mst_helper.h
> > > +++ b/include/drm/drm_dp_mst_helper.h
> > > @@ -406,9 +406,16 @@ struct drm_dp_payload {
> > >  
> > >  #define to_dp_mst_topology_state(x) container_of(x, struct
> > > drm_dp_mst_topology_state, base)
> > >  
> > > +struct drm_dp_vcpi_allocation {
> > 
> > I think a comment here that this pointer holds a full reference (even past
> > when the connector is unplugged) would be good. Otherwise the loop in
> > release_vcpi would be buggy.
> > 
> > > +	struct drm_dp_mst_port *port;
> > > +	int vcpi;
> > > +	struct list_head next;
> > > +};
> > > +
> > >  struct drm_dp_mst_topology_state {
> > >  	struct drm_private_state base;
> > > -	int avail_slots;
> > > +	struct list_head vcpis;
> > > +	u8 vcpi_allocated;
> > >  	struct drm_dp_mst_topology_mgr *mgr;
> > >  };
> > >  
> > > @@ -619,14 +626,17 @@ void drm_dp_mst_topology_mgr_suspend(struct
> > > drm_dp_mst_topology_mgr *mgr);
> > >  int drm_dp_mst_topology_mgr_resume(struct drm_dp_mst_topology_mgr *mgr);
> > >  struct drm_dp_mst_topology_state
> > > *drm_atomic_get_mst_topology_state(struct drm_atomic_state *state,
> > >  								    struct
> > > drm_dp_mst_topology_mgr *mgr);
> > > -int drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state *state,
> > > -				  struct drm_dp_mst_topology_mgr *mgr,
> > > -				  struct drm_dp_mst_port *port, int pbn);
> > > -int drm_dp_atomic_release_vcpi_slots(struct drm_atomic_state *state,
> > > -				     struct drm_dp_mst_topology_mgr *mgr,
> > > -				     int slots);
> > > +int __must_check
> > > +drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state *state,
> > > +			      struct drm_dp_mst_topology_mgr *mgr,
> > > +			      struct drm_dp_mst_port *port, int pbn);
> > > +int __must_check
> > > +drm_dp_atomic_release_vcpi_slots(struct drm_atomic_state *state,
> > > +				 struct drm_dp_mst_topology_mgr *mgr,
> > > +				 struct drm_dp_mst_port *port);
> > >  int drm_dp_send_power_updown_phy(struct drm_dp_mst_topology_mgr *mgr,
> > >  				 struct drm_dp_mst_port *port, bool power_up);
> > > +int __must_check drm_dp_mst_atomic_check(struct drm_atomic_state *state);
> > >  
> > >  extern const struct drm_private_state_funcs
> > > drm_dp_mst_topology_state_funcs;
> > >  
> > > -- 
> > > 2.19.1
> > > 
> -- 
> Cheers,
> 	Lyude Paul
>

Lyude Paul Nov. 27, 2018, 5:48 p.m. UTC | #5

On Mon, 2018-11-26 at 22:22 +0100, Daniel Vetter wrote:
> On Mon, Nov 26, 2018 at 10:04:21PM +0100, Daniel Vetter wrote:
> > On Thu, Nov 15, 2018 at 07:50:05PM -0500, Lyude Paul wrote:
> > > There has been a TODO waiting for quite a long time in
> > > drm_dp_mst_topology.c:
> > > 
> > > 	/* We cannot rely on port->vcpi.num_slots to update
> > > 	 * topology_state->avail_slots as the port may not exist if the parent
> > > 	 * branch device was unplugged. This should be fixed by tracking
> > > 	 * per-port slot allocation in drm_dp_mst_topology_state instead of
> > > 	 * depending on the caller to tell us how many slots to release.
> > > 	 */
> > > 
> > > That's not the only reason we should fix this: forcing the driver to
> > > track the VCPI allocations throughout a state's atomic check is
> > > error prone, because it means that extra care has to be taken with the
> > > order that drm_dp_atomic_find_vcpi_slots() and
> > > drm_dp_atomic_release_vcpi_slots() are called in in order to ensure
> > > idempotency. Currently the only driver actually using these helpers,
> > > i915, doesn't even do this correctly: multiple ->best_encoder() checks
> > > with i915's current implementation would not be idempotent and would
> > > over-allocate VCPI slots, something I learned trying to implement
> > > fallback retraining in MST.
> > > 
> > > So: simplify this whole mess, and teach drm_dp_atomic_find_vcpi_slots()
> > > and drm_dp_atomic_release_vcpi_slots() to track the VCPI allocations for
> > > each port. This allows us to ensure idempotency without having to rely
> > > on the driver as much. Additionally: the driver doesn't need to do any
> > > kind of VCPI slot tracking anymore if it doesn't need it for it's own
> > > internal state.
> > > 
> > > Additionally; this adds a new drm_dp_mst_atomic_check() helper which
> > > must be used by atomic drivers to perform validity checks for the new
> > > VCPI allocations incurred by a state.
> > > 
> > > Also: update the documentation and make it more obvious that these
> > > /must/ be called by /all/ atomic drivers supporting MST.
> > > 
> > > Changes since v6:
> > >  - Keep a kref to all of the ports we have allocations on. This required
> > >    a good bit of changing to when we call drm_dp_find_vcpi_slots(),
> > >    mainly that we need to ensure that we only redo VCPI allocations on
> > >    actual mode or CRTC changes, not crtc_state->active changes.
> > >    Additionally, we no longer take the registration of the DRM connector
> > >    for each port into account because so long as we have a kref to the
> > >    port in the new or previous atomic state, the connector will stay
> > >    registered.
> > 
> > I write an entire pile of small nitpits (still included most of them
> > below), until I realized this here wont work. Delaying the call to destroy
> > the connector (well, unregister it really) wreaks the design we've come up
> > with thus far, resulting in most of my comments here.
> > 
> > Instead, all we need to do is delay the kfree(port) at the bottom of
> > drm_dp_destroy_port(). The vcpi allocation structure _only_ needs the
> > pointer value to stay valid, as a lookup key. It doesn't care at all about
> > anything actually stored in there. So the only thing we need to delay is
> > the kfree. I think the simplest way to achieve this is to add a 2nd kref
> > (port->kfree_ref or something like that), with on reference held by the
> > port itself (released in drm_dp_destroy_port), and the other one held
> > as-needed by the vcpi allocation lists.
> > 
> > I think if we go with this design instead of retrofitting a semantic
> > change of the port lifetime itself, all the complications I complain about
> > below should disappear.
> 
> In the above I meant drm_dp_destroy_port or drm_dp_destroy_connector_work.
> 
> Aside: I think creating a kref for the final kfree would also solve a
> bunch of other issues in a much neater way: In
> 
> commit f038c5b99fc1332f558b495d136d4f433ee65caa
> Author: Lyude Paul <[e-mail]>
> Date:   Tue Nov 13 17:46:14 2018 -0500
> 
>     drm/dp_mst: Skip validating ports during destruction, just ref
> 
> we could use that kfree reference to make sure the port pointer is alive.
> This of course means that drm_dp_update_payload_part1() would also need to
> use the kfree reference for the vcpi allocations. And probably everywhere
> else (e.g. in your nouveau series for payload information).
> 
> That would give us a very clear separation overall between "port can still
> be used because it's not yet unplugged" vs. "port only hangs around
> because a bunch of vcpi allocations and other payload things upstream of
> the port might still need the port lookup key to free resources". Instead
> of again sprinkling complicated conditions and magic tricks all over the
> code to figure out whethe we should allow the get_validate_port to succeed
> or not, or the modeset to succeed or not.

mhm, I've been experimenting with this and I think I agree this is definitely
the right solution to go with. It'll end up decomplicating a lot of this :)
> -Daniel
> 
> > Piles of comments below.
> > 
> > Cheers, Daniel
> > 
> > >  - Use the small changes to drm_dp_put_port() to add even more error
> > >    checking to make misusage of the helpers more obvious. I added this
> > >    after having to chase down various use-after-free conditions that
> > >    started popping up from the new helpers so no one else has to
> > >    troubleshoot that.
> > >  - Move some accidental DRM_DEBUG_KMS() calls to DRM_DEBUG_ATOMIC()
> > >  - Update documentation again, note that find/release() should both not
> > > be
> > >    called on the same port in a single atomic check phase (but multiple
> > >    calls to one or the other is OK)
> > > 
> > > Changes since v4:
> > >  - Don't skip the atomic checks for VCPI allocations if no new VCPI
> > >    allocations happen in a state. This makes the next change I'm about
> > >    to list here a lot easier to implement.
> > >  - Don't ignore VCPI allocations on destroyed ports, instead ensure that
> > >    when ports are destroyed and still have VCPI allocations in the
> > >    topology state, the only state changes allowed are releasing said
> > >    ports' VCPI. This prevents a state with a mix of VCPI allocations
> > >    from destroyed ports, and allocations from valid ports.
> > > 
> > > Changes since v3:
> > >  - Don't release VCPI allocations in the topology state immediately in
> > >    drm_dp_atomic_release_vcpi_slots(), instead mark them as 0 and skip
> > >    over them in drm_dp_mst_duplicate_state(). This makes it so
> > >    drm_dp_atomic_release_vcpi_slots() is still idempotent while also
> > >    throwing warnings if the driver messes up it's book keeping and tries
> > >    to release VCPI slots on a port that doesn't have any pre-existing
> > >    VCPI allocation - danvet
> > >  - Change mst_state/state in some debugging messages to "mst state"
> > > 
> > > Changes since v2:
> > >  - Use kmemdup() for duplicating MST state - danvet
> > >  - Move port validation out of duplicate state callback - danvet
> > >  - Handle looping through MST topology states in
> > >    drm_dp_mst_atomic_check() so the driver doesn't have to do it
> > >  - Fix documentation in drm_dp_atomic_find_vcpi_slots()
> > >  - Move the atomic check for each individual topology state into it's
> > >    own function, reduces indenting
> > >  - Don't consider "stale" MST ports when calculating the bandwidth
> > >    requirements. This is needed because originally we relied on the
> > >    state duplication functions to prune any stale ports from the new
> > >    state, which would prevent us from incorrectly considering their
> > >    bandwidth requirements alongside legitimate new payloads.
> > >  - Add function references in drm_dp_atomic_release_vcpi_slots() -
> > > danvet
> > >  - Annotate atomic VCPI and atomic check functions with __must_check
> > >    - danvet
> > > 
> > > Changes since v1:
> > >  - Don't use the now-removed ->atomic_check() for private objects hook,
> > >    just give drivers a function to call themselves
> > > 
> > > Signed-off-by: Lyude Paul <[e-mail]>
> > > Cc: Daniel Vetter <[e-mail]>
> > > ---
> > >  drivers/gpu/drm/drm_dp_mst_topology.c | 294 +++++++++++++++++++++++---
> > >  drivers/gpu/drm/i915/intel_display.c  |   4 +
> > >  drivers/gpu/drm/i915/intel_dp_mst.c   |  64 +++---
> > >  include/drm/drm_dp_mst_helper.h       |  24 ++-
> > >  4 files changed, 319 insertions(+), 67 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c
> > > b/drivers/gpu/drm/drm_dp_mst_topology.c
> > > index 00fbe7a2699d..183d0e832ced 100644
> > > --- a/drivers/gpu/drm/drm_dp_mst_topology.c
> > > +++ b/drivers/gpu/drm/drm_dp_mst_topology.c
> > > @@ -2617,21 +2617,42 @@ static int drm_dp_init_vcpi(struct
> > > drm_dp_mst_topology_mgr *mgr,
> > >  }
> > >  
> > >  /**
> > > - * drm_dp_atomic_find_vcpi_slots() - Find and add vcpi slots to the
> > > state
> > > + * drm_dp_atomic_find_vcpi_slots() - Find and add VCPI slots to the
> > > state
> > >   * @state: global atomic state
> > >   * @mgr: MST topology manager for the port
> > >   * @port: port to find vcpi slots for
> > >   * @pbn: bandwidth required for the mode in PBN
> > >   *
> > > - * RETURNS:
> > > - * Total slots in the atomic state assigned for this port or error
> > > + * Allocates VCPI slots to @port, replacing any previous VCPI
> > > allocations it
> > > + * may have had. Any atomic drivers which support MST must call this
> > > function
> > > + * in their &drm_encoder_helper_funcs.atomic_check() callback to change
> > > the
> > > + * current VCPI allocation for the new state, but only when
> > > + * &drm_crtc_state.mode_changed or &drm_crtc_state.connectors_changed
> > > is set
> > > + * to ensure compatibility with userspace applications that still use
> > > the
> > > + * legacy modesetting UAPI.
> > > + *
> > > + * Allocations set by this function are not checked against the
> > > bandwidth
> > > + * restraints of @mgr until the driver calls drm_dp_mst_atomic_check().
> > > + *
> > > + * Additionally, it is OK to call this function multiple times on the
> > > same
> > > + * @port as needed. It is not OK however, to call this function and
> > > + * drm_dp_atomic_release_vcpi_slots() in the same atomic check phase.
> > > + *
> > > + * See also:
> > > + * drm_dp_atomic_release_vcpi_slots()
> > > + * drm_dp_mst_atomic_check()
> > > + *
> > > + * Returns:
> > > + * Total slots in the atomic state assigned for this port, or a
> > > negative error
> > > + * code if the port no longer exists
> > >   */
> > >  int drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state *state,
> > >  				  struct drm_dp_mst_topology_mgr *mgr,
> > >  				  struct drm_dp_mst_port *port, int pbn)
> > >  {
> > >  	struct drm_dp_mst_topology_state *topology_state;
> > > -	int req_slots;
> > > +	struct drm_dp_vcpi_allocation *pos, *vcpi = NULL;
> > > +	int prev_slots, req_slots, ret;
> > >  
> > >  	topology_state = drm_atomic_get_mst_topology_state(state, mgr);
> > >  	if (IS_ERR(topology_state))
> > > @@ -2640,20 +2661,60 @@ int drm_dp_atomic_find_vcpi_slots(struct
> > > drm_atomic_state *state,
> > >  	port = drm_dp_get_validated_port_ref(mgr, port);
> > >  	if (port == NULL)
> > >  		return -EINVAL;
> > > -	req_slots = DIV_ROUND_UP(pbn, mgr->pbn_div);
> > > -	DRM_DEBUG_KMS("vcpi slots req=%d, avail=%d\n",
> > > -			req_slots, topology_state->avail_slots);
> > >  
> > > -	if (req_slots > topology_state->avail_slots) {
> > > -		drm_dp_put_port(port);
> > > -		return -ENOSPC;
> > > +	topology_state->vcpi_allocated = true;
> > > +
> > > +	/* Find the current allocation for this port, if any */
> > > +	list_for_each_entry(pos, &topology_state->vcpis, next) {
> > > +		if (pos->port == port) {
> > > +			vcpi = pos;
> > > +			prev_slots = vcpi->vcpi;
> > > +
> > > +			/*
> > > +			 * This should never happen, unless the driver tries
> > > +			 * releasing and allocating the same VCPI allocation,
> > > +			 * which is an error
> > > +			 */
> > > +			if (WARN_ON(!prev_slots)) {
> > > +				DRM_ERROR("cannot allocate and release VCPI on
> > > [MST PORT:%p] in the same state\n",
> > > +					  port);
> > > +				return -EINVAL;
> > > +			}
> > 
> > Allowing atomic changes to output state (like disabling it on one crtc,
> > and then enabling on the other, thus both releasing and re-allocating the
> > vcpi in one go) is kinda the point of atomic. So definitely can't EINVAL
> > here.
> > 
> > Sounds like we need more testcases in igt?
> > 
> > > +
> > > +			break;
> > > +		}
> > >  	}
> > > +	if (!vcpi)
> > > +		prev_slots = 0;
> > >  
> > > -	topology_state->avail_slots -= req_slots;
> > > -	DRM_DEBUG_KMS("vcpi slots avail=%d", topology_state->avail_slots);
> > > +	req_slots = DIV_ROUND_UP(pbn, mgr->pbn_div);
> > > +
> > > +	DRM_DEBUG_ATOMIC("[CONNECTOR:%d:%s] [MST PORT:%p] VCPI %d -> %d\n",
> > > +			 port->connector->base.id, port->connector->name,
> > > +			 port, prev_slots, req_slots);
> > > +
> > > +	/* Add the new allocation to the state */
> > > +	if (!vcpi) {
> > > +		vcpi = kzalloc(sizeof(*vcpi), GFP_KERNEL);
> > > +		if (!vcpi) {
> > > +			ret = -ENOMEM;
> > > +			goto out;
> > > +		}
> > >  
> > > +		vcpi->port = drm_dp_get_validated_port_ref(mgr, port);
> > > +		if (!vcpi->port) {
> > > +			ret = -EINVAL;
> > > +			kfree(vcpi);
> > > +			goto out;
> > > +		}
> > > +		list_add(&vcpi->next, &topology_state->vcpis);
> > > +	}
> > > +	vcpi->vcpi = req_slots;
> > > +
> > > +	ret = req_slots;
> > > +out:
> > >  	drm_dp_put_port(port);
> > > -	return req_slots;
> > > +	return ret;
> > >  }
> > >  EXPORT_SYMBOL(drm_dp_atomic_find_vcpi_slots);
> > >  
> > > @@ -2661,31 +2722,66 @@ EXPORT_SYMBOL(drm_dp_atomic_find_vcpi_slots);
> > >   * drm_dp_atomic_release_vcpi_slots() - Release allocated vcpi slots
> > >   * @state: global atomic state
> > >   * @mgr: MST topology manager for the port
> > > - * @slots: number of vcpi slots to release
> > > + * @port: The port to release the VCPI slots from
> > >   *
> > > - * RETURNS:
> > > - * 0 if @slots were added back to &drm_dp_mst_topology_state-
> > > >avail_slots or
> > > - * negative error code
> > > + * Releases any VCPI slots that have been allocated to a port in the
> > > atomic
> > > + * state. Any atomic drivers which support MST must call this function
> > > in
> > > + * their &drm_connector_helper_funcs.atomic_check() callback when the
> > > + * connector will no longer have VCPI allocated (e.g. because it's CRTC
> > > was
> > > + * removed) when it had VCPI allocated in the previous atomic state.
> > > + *
> > > + * It is OK to call this even if @port has been removed from the
> > > system.
> > > + * Additionally, it is OK to call this function multiple times on the
> > > same
> > > + * @port as needed. It is not OK however, to call this function and
> > > + * drm_dp_atomic_find_vcpi_slots() on the same @port in a single atomic
> > > check
> > > + * phase.
> > > + *
> > > + * See also:
> > > + * drm_dp_atomic_find_vcpi_slots()
> > > + * drm_dp_mst_atomic_check()
> > > + *
> > > + * Returns:
> > > + * 0 if all slots for this port were added back to
> > > + * &drm_dp_mst_topology_state.avail_slots or negative error code
> > >   */
> > >  int drm_dp_atomic_release_vcpi_slots(struct drm_atomic_state *state,
> > >  				     struct drm_dp_mst_topology_mgr *mgr,
> > > -				     int slots)
> > > +				     struct drm_dp_mst_port *port)
> > >  {
> > >  	struct drm_dp_mst_topology_state *topology_state;
> > > +	struct drm_dp_vcpi_allocation *pos;
> > > +	bool found = false;
> > >  
> > >  	topology_state = drm_atomic_get_mst_topology_state(state, mgr);
> > >  	if (IS_ERR(topology_state))
> > >  		return PTR_ERR(topology_state);
> > >  
> > > -	/* We cannot rely on port->vcpi.num_slots to update
> > > -	 * topology_state->avail_slots as the port may not exist if the parent
> > > -	 * branch device was unplugged. This should be fixed by tracking
> > > -	 * per-port slot allocation in drm_dp_mst_topology_state instead of
> > > -	 * depending on the caller to tell us how many slots to release.
> > > -	 */
> > > -	topology_state->avail_slots += slots;
> > > -	DRM_DEBUG_KMS("vcpi slots released=%d, avail=%d\n",
> > > -			slots, topology_state->avail_slots);
> > > +	list_for_each_entry(pos, &topology_state->vcpis, next) {
> > > +		if (pos->port == port) {
> > > +			found = true;
> > > +			break;
> > > +		}
> > > +	}
> > > +	if (WARN_ON(!found)) {
> > > +		DRM_ERROR("no VCPI for [MST PORT:%p] found in mst state %p\n",
> > > +			  port, &topology_state->base);
> > > +		return -EINVAL;
> > > +	}
> > > +
> > > +	DRM_DEBUG_ATOMIC("[MST PORT:%p] VCPI %d -> 0\n", port, pos->vcpi);
> > > +	if (pos->vcpi) {
> > > +		/*
> > > +		 * This should be safe-as the previous state will still hold a
> > > +		 * reference to this port until we swap states. If it's not,
> > > +		 * the driver has a bookkeeping error
> > > +		 */
> > > +		if (WARN_ON(drm_dp_put_port(port))) {
> > 
> > The usual way to do this (instead of adding a return value to the normal
> > _put function) is something like
> > 
> > 	kref_put(port, function_that_with_just_a_WARNING_DRM_ERROR)
> > 
> > avoids the need for patch 2.
> > 
> > > +			DRM_ERROR("cannot allocate and release VCPI on [MST
> > > PORT:%p] in the same state\n",
> > > +				  port);
> > > +			return -EINVAL;
> > > +		}
> > > +		pos->vcpi = 0;
> > > +	}
> > >  
> > >  	return 0;
> > >  }
> > > @@ -3115,15 +3211,45 @@ static void drm_dp_destroy_connector_work(struct
> > > work_struct *work)
> > >  static struct drm_private_state *
> > >  drm_dp_mst_duplicate_state(struct drm_private_obj *obj)
> > >  {
> > > -	struct drm_dp_mst_topology_state *state;
> > > +	struct drm_dp_mst_topology_state *state, *old_state =
> > > +		to_dp_mst_topology_state(obj->state);
> > > +	struct drm_dp_vcpi_allocation *pos, *vcpi;
> > >  
> > > -	state = kmemdup(obj->state, sizeof(*state), GFP_KERNEL);
> > > +	state = kmemdup(old_state, sizeof(*state), GFP_KERNEL);
> > >  	if (!state)
> > >  		return NULL;
> > >  
> > >  	__drm_atomic_helper_private_obj_duplicate_state(obj, &state->base);
> > >  
> > > +	INIT_LIST_HEAD(&state->vcpis);
> > > +	state->vcpi_allocated = false;
> > > +
> > > +	list_for_each_entry(pos, &old_state->vcpis, next) {
> > > +		/* Prune leftover freed VCPI allocations */
> > > +		if (!pos->vcpi)
> > > +			continue;
> > > +
> > > +		vcpi = kmemdup(pos, sizeof(*vcpi), GFP_KERNEL);
> > > +		if (!vcpi)
> > > +			goto fail;
> > > +
> > > +		/* Take a plain old kref here and don't validate the port, as
> > > +		 * it's topology might not even exist anymore
> > > +		 */
> > > +		kref_get(&vcpi->port->kref);
> > > +		list_add(&vcpi->next, &state->vcpis);
> > > +	}
> > > +
> > >  	return &state->base;
> > > +
> > > +fail:
> > > +	list_for_each_entry_safe(pos, vcpi, &state->vcpis, next) {
> > > +		drm_dp_put_port(pos->port);
> > > +		kfree(pos);
> > > +	}
> > > +	kfree(state);
> > > +
> > > +	return NULL;
> > >  }
> > >  
> > >  static void drm_dp_mst_destroy_state(struct drm_private_obj *obj,
> > > @@ -3131,14 +3257,116 @@ static void drm_dp_mst_destroy_state(struct
> > > drm_private_obj *obj,
> > >  {
> > >  	struct drm_dp_mst_topology_state *mst_state =
> > >  		to_dp_mst_topology_state(state);
> > > +	struct drm_dp_vcpi_allocation *pos, *tmp;
> > > +
> > > +	list_for_each_entry_safe(pos, tmp, &mst_state->vcpis, next) {
> > > +		/* We only keep references to ports with non-zero VCPIs */
> > > +		if (pos->vcpi)
> > > +			drm_dp_put_port(pos->port);
> > > +		kfree(pos);
> > > +	}
> > >  
> > >  	kfree(mst_state);
> > >  }
> > >  
> > > -static const struct drm_private_state_funcs mst_state_funcs = {
> > > +static inline int
> > > +drm_dp_mst_atomic_check_topology_state(struct drm_dp_mst_topology_mgr
> > > *mgr,
> > > +				       struct drm_dp_mst_topology_state
> > > *mst_state)
> > > +{
> > > +	struct drm_dp_vcpi_allocation *vcpi;
> > > +	struct drm_dp_mst_port *port;
> > > +	int avail_slots = 63, ret;
> > > +
> > > +	/* There's no possible scenario where releasing VCPI or keeping it the
> > > +	 * same would make the state invalid
> > > +	 */
> > > +	if (!mst_state->vcpi_allocated) {
> > 
> > Do we need this? If not I'm always in favour of removing code if it's not
> > needed :-)
> > 
> > > +		DRM_DEBUG_ATOMIC("[MST MGR:%p] state %p allocates no VCPI,
> > > check skipped\n",
> > > +				 mgr, &mst_state->base);
> > > +		return 0;
> > > +	}
> > > +
> > > +	list_for_each_entry(vcpi, &mst_state->vcpis, next) {
> > > +		/* Releasing VCPI is always OK-even if the port is gone */
> > > +		if (!vcpi->vcpi) {
> > > +			DRM_DEBUG_ATOMIC("[MST PORT:%p] releases all VCPI
> > > slots\n",
> > > +					 vcpi->port);
> > > +			continue;
> > > +		}
> > > +
> > > +		port = drm_dp_get_validated_port_ref(mgr, vcpi->port);
> > > +		if (!port) {
> > > +			DRM_DEBUG_ATOMIC("[MST PORT:%p] is gone but still has
> > > VCPI, cannot add new VCPI\n",
> > > +					 vcpi->port);
> > > +			return -EINVAL;
> > > +		}
> > 
> > For find_vcpi we've already done this. For unplugged vcpi, we can't do
> > this here I think - otherwise if you enable 2 mst outputs on the same
> > branch, and unplug both, old non-atomic userspace can never ever again
> > disable them because it can't disable both. The check (to skip already
> > deallocated ports) above isn't enough to handle that.
> > 
> > > +
> > > +		DRM_DEBUG_ATOMIC("[MST PORT:%p] requires %d vcpi slots\n",
> > > +				 vcpi->port, vcpi->vcpi);
> > > +
> > > +		avail_slots -= vcpi->vcpi;
> > > +		if (avail_slots < 0) {
> > > +			DRM_DEBUG_ATOMIC("[MST PORT:%p] not enough VCPI slots
> > > in mst state %p (avail=%d)\n",
> > > +					 vcpi->port, mst_state,
> > > +					 avail_slots + vcpi->vcpi);
> > > +			ret = -ENOSPC;
> > > +			goto port_fail;
> > > +		}
> > > +
> > > +		drm_dp_put_port(port);
> > > +	}
> > > +	DRM_DEBUG_ATOMIC("[MST MGR:%p] mst state %p VCPI avail=%d used=%d\n",
> > > +			 mgr, mst_state, avail_slots,
> > > +			 63 - avail_slots);
> > > +
> > > +	return 0;
> > > +port_fail:
> > > +	drm_dp_put_port(port);
> > > +	return ret;
> > > +}
> > > +
> > > +/**
> > > + * drm_dp_mst_atomic_check - Check that the new state of an MST
> > > topology in an
> > > + * atomic update is valid
> > > + * @state: Pointer to the new &struct drm_dp_mst_topology_state
> > > + *
> > > + * Checks the given topology state for an atomic update to ensure that
> > > it's
> > > + * valid. This includes checking whether there's enough bandwidth to
> > > support
> > > + * the new VCPI allocations in the atomic update.
> > > + *
> > > + * Any atomic drivers supporting DP MST must make sure to call this
> > > after
> > > + * checking the rest of their state in their
> > > + * &drm_mode_config_funcs.atomic_check() callback.
> > > + *
> > > + * See also:
> > > + * drm_dp_atomic_find_vcpi_slots()
> > > + * drm_dp_atomic_release_vcpi_slots()
> > > + *
> > > + * Returns:
> > > + *
> > > + * 0 if the new state is valid, negative error code otherwise.
> > > + */
> > > +int drm_dp_mst_atomic_check(struct drm_atomic_state *state)
> > > +{
> > > +	struct drm_dp_mst_topology_mgr *mgr;
> > > +	struct drm_dp_mst_topology_state *mst_state;
> > > +	int i, ret = 0;
> > > +
> > > +	for_each_new_mst_mgr_in_state(state, mgr, mst_state, i) {
> > > +		ret = drm_dp_mst_atomic_check_topology_state(mgr, mst_state);
> > > +		if (ret)
> > > +			break;
> > > +	}
> > > +
> > > +	return ret;
> > > +}
> > > +EXPORT_SYMBOL(drm_dp_mst_atomic_check);
> > > +
> > > +const struct drm_private_state_funcs drm_dp_mst_topology_state_funcs =
> > > {
> > >  	.atomic_duplicate_state = drm_dp_mst_duplicate_state,
> > >  	.atomic_destroy_state = drm_dp_mst_destroy_state,
> > >  };
> > > +EXPORT_SYMBOL(drm_dp_mst_topology_state_funcs);
> > >  
> > >  /**
> > >   * drm_atomic_get_mst_topology_state: get MST topology state
> > > @@ -3216,13 +3444,11 @@ int drm_dp_mst_topology_mgr_init(struct
> > > drm_dp_mst_topology_mgr *mgr,
> > >  		return -ENOMEM;
> > >  
> > >  	mst_state->mgr = mgr;
> > > -
> > > -	/* max. time slots - one slot for MTP header */
> > > -	mst_state->avail_slots = 63;
> > > +	INIT_LIST_HEAD(&mst_state->vcpis);
> > >  
> > >  	drm_atomic_private_obj_init(&mgr->base,
> > >  				    &mst_state->base,
> > > -				    &mst_state_funcs);
> > > +				    &drm_dp_mst_topology_state_funcs);
> > >  
> > >  	return 0;
> > >  }
> > > diff --git a/drivers/gpu/drm/i915/intel_display.c
> > > b/drivers/gpu/drm/i915/intel_display.c
> > > index 132e978227fb..8e85e04a5948 100644
> > > --- a/drivers/gpu/drm/i915/intel_display.c
> > > +++ b/drivers/gpu/drm/i915/intel_display.c
> > > @@ -12592,6 +12592,10 @@ static int intel_atomic_check(struct drm_device
> > > *dev,
> > >  				       "[modeset]" : "[fastset]");
> > >  	}
> > >  
> > > +	ret = drm_dp_mst_atomic_check(state);
> > > +	if (ret)
> > > +		return ret;
> > > +
> > >  	if (any_ms) {
> > >  		ret = intel_modeset_checks(state);
> > >  
> > > diff --git a/drivers/gpu/drm/i915/intel_dp_mst.c
> > > b/drivers/gpu/drm/i915/intel_dp_mst.c
> > > index 4de247ddf05f..be2d1ac7d87d 100644
> > > --- a/drivers/gpu/drm/i915/intel_dp_mst.c
> > > +++ b/drivers/gpu/drm/i915/intel_dp_mst.c
> > > @@ -41,8 +41,13 @@ static bool intel_dp_mst_compute_config(struct
> > > intel_encoder *encoder,
> > >  	struct drm_connector *connector = conn_state->connector;
> > >  	void *port = to_intel_connector(connector)->port;
> > >  	struct drm_atomic_state *state = pipe_config->base.state;
> > > +	struct drm_crtc *crtc = pipe_config->base.crtc;
> > > +	struct drm_crtc_state *old_crtc_state =
> > > +		drm_atomic_get_old_crtc_state(state, crtc);
> > > +	struct drm_crtc_state *new_crtc_state = &pipe_config->base;
> > >  	int bpp;
> > > -	int lane_count, slots = 0;
> > > +	int lane_count, slots =
> > > +		to_intel_crtc_state(old_crtc_state)->dp_m_n.tu;
> > >  	const struct drm_display_mode *adjusted_mode = &pipe_config-
> > > >base.adjusted_mode;
> > >  	int mst_pbn;
> > >  	bool constant_n = drm_dp_has_quirk(&intel_dp->desc,
> > > @@ -77,8 +82,12 @@ static bool intel_dp_mst_compute_config(struct
> > > intel_encoder *encoder,
> > >  	mst_pbn = drm_dp_calc_pbn_mode(adjusted_mode->crtc_clock, bpp);
> > >  	pipe_config->pbn = mst_pbn;
> > >  
> > > -	/* Zombie connectors can't have VCPI slots */
> > > -	if (!drm_connector_is_unregistered(connector)) {
> > > +	/* Only change VCPI allocation on actual mode changes, to prevent us
> > > +	 * from trying to allocate VCPI to ports that no longer exist when we
> > > +	 * may just be trying to disable DPMS on them
> > > +	 */
> > > +	if (new_crtc_state->mode_changed ||
> > > +	    new_crtc_state->connectors_changed) {
> > 
> > Why exactly is this needed? We only call compute_config on the enocoder
> > when enabling an output. Not when disabling one. Similar for the atomic
> > helpers and the various encoder callbacks (you have the same change in the
> > nouveau code). Did this actually blow up while testing?
> > 
> > >  		slots = drm_dp_atomic_find_vcpi_slots(state,
> > >  						      &intel_dp->mst_mgr,
> > >  						      port,
> > > @@ -107,36 +116,39 @@ static bool intel_dp_mst_compute_config(struct
> > > intel_encoder *encoder,
> > >  	return true;
> > >  }
> > >  
> > > -static int intel_dp_mst_atomic_check(struct drm_connector *connector,
> > > -		struct drm_connector_state *new_conn_state)
> > > +static int
> > > +intel_dp_mst_atomic_check(struct drm_connector *connector,
> > > +			  struct drm_connector_state *new_conn_state)
> > >  {
> > > +	struct intel_connector *intel_connector =
> > > +		to_intel_connector(connector);
> > > +	struct drm_dp_mst_topology_mgr *mgr =
> > > +		&intel_connector->mst_port->mst_mgr;
> > > +	struct drm_dp_mst_port *port = intel_connector->port;
> > >  	struct drm_atomic_state *state = new_conn_state->state;
> > > -	struct drm_connector_state *old_conn_state;
> > > -	struct drm_crtc *old_crtc;
> > > -	struct drm_crtc_state *crtc_state;
> > > -	int slots, ret = 0;
> > > +	struct drm_connector_state *old_conn_state =
> > > +		drm_atomic_get_old_connector_state(state, connector);
> > > +	struct drm_crtc *old_crtc = old_conn_state->crtc,
> > > +			*new_crtc = new_conn_state->crtc;
> > > +	struct drm_crtc_state *old_crtc_state, *new_crtc_state;
> > >  
> > > -	old_conn_state = drm_atomic_get_old_connector_state(state, connector);
> > > -	old_crtc = old_conn_state->crtc;
> > 
> > Bikeshed: Reworking all the above hides the real changes quite a bit.
> > 
> > >  	if (!old_crtc)
> > > -		return ret;
> > > +		return 0;
> > >  
> > > -	crtc_state = drm_atomic_get_new_crtc_state(state, old_crtc);
> > > -	slots = to_intel_crtc_state(crtc_state)->dp_m_n.tu;
> > > -	if (drm_atomic_crtc_needs_modeset(crtc_state) && slots > 0) {
> > > -		struct drm_dp_mst_topology_mgr *mgr;
> > > -		struct drm_encoder *old_encoder;
> > > +	old_crtc_state = drm_atomic_get_old_crtc_state(state, old_crtc);
> > > +	if (!old_crtc_state ||
> > 
> > !old_crtc_state would be a framework bug. Just remove that check.
> > 
> > > +	    !to_intel_crtc_state(old_crtc_state)->dp_m_n.tu)
> > > +		return 0;
> > 
> > Hm, why do we need this? Having a DP port previously enabled with a 0 tu
> > sounds like a pretty big driver bug.
> > >  
> > > -		old_encoder = old_conn_state->best_encoder;
> > > -		mgr = &enc_to_mst(old_encoder)->primary->dp.mst_mgr;
> > > +	/* If we switched CRTCs, clear the previous one's allocation */
> > > +	new_crtc_state = drm_atomic_get_new_crtc_state(state, old_crtc);
> > > +	if (new_crtc_state->connectors_changed && !new_crtc_state->enable)
> > > +		to_intel_crtc_state(new_crtc_state)->dp_m_n.tu = 0;
> > 
> > No idea why we need to clear this here? Shouldn't we just clear this
> > unconditionally if we release the allocation.
> > 
> > Also naming convention, I'd call this old_crtc_new_state, and maybe
> > old_crtc_old_state. To make it clear these are the old/new states for
> > old_crtc. Instead of old/new state for the new crtc.
> > 
> > >  
> > > -		ret = drm_dp_atomic_release_vcpi_slots(state, mgr, slots);
> > > -		if (ret)
> > > -			DRM_DEBUG_KMS("failed releasing %d vcpi slots:%d\n",
> > > slots, ret);
> > > -		else
> > > -			to_intel_crtc_state(crtc_state)->dp_m_n.tu = 0;
> > > -	}
> > > -	return ret;
> > > +	if (new_crtc)
> > > +		return 0;
> > 
> > Not sure why we shouldn't release this? If we handle this in helpers, it
> > would remove piles of special casing code here I think. Plus remove
> > special code in the hlpers.
> > 
> > 
> > > +
> > > +	return drm_dp_atomic_release_vcpi_slots(state, mgr, port);
> > >  }
> > >  
> > >  static void intel_mst_disable_dp(struct intel_encoder *encoder,
> > > diff --git a/include/drm/drm_dp_mst_helper.h
> > > b/include/drm/drm_dp_mst_helper.h
> > > index 3faceb66f5cb..2392e12b6bce 100644
> > > --- a/include/drm/drm_dp_mst_helper.h
> > > +++ b/include/drm/drm_dp_mst_helper.h
> > > @@ -406,9 +406,16 @@ struct drm_dp_payload {
> > >  
> > >  #define to_dp_mst_topology_state(x) container_of(x, struct
> > > drm_dp_mst_topology_state, base)
> > >  
> > > +struct drm_dp_vcpi_allocation {
> > 
> > I think a comment here that this pointer holds a full reference (even past
> > when the connector is unplugged) would be good. Otherwise the loop in
> > release_vcpi would be buggy.
> > 
> > > +	struct drm_dp_mst_port *port;
> > > +	int vcpi;
> > > +	struct list_head next;
> > > +};
> > > +
> > >  struct drm_dp_mst_topology_state {
> > >  	struct drm_private_state base;
> > > -	int avail_slots;
> > > +	struct list_head vcpis;
> > > +	u8 vcpi_allocated;
> > >  	struct drm_dp_mst_topology_mgr *mgr;
> > >  };
> > >  
> > > @@ -619,14 +626,17 @@ void drm_dp_mst_topology_mgr_suspend(struct
> > > drm_dp_mst_topology_mgr *mgr);
> > >  int drm_dp_mst_topology_mgr_resume(struct drm_dp_mst_topology_mgr
> > > *mgr);
> > >  struct drm_dp_mst_topology_state
> > > *drm_atomic_get_mst_topology_state(struct drm_atomic_state *state,
> > >  								    struct
> > > drm_dp_mst_topology_mgr *mgr);
> > > -int drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state *state,
> > > -				  struct drm_dp_mst_topology_mgr *mgr,
> > > -				  struct drm_dp_mst_port *port, int pbn);
> > > -int drm_dp_atomic_release_vcpi_slots(struct drm_atomic_state *state,
> > > -				     struct drm_dp_mst_topology_mgr *mgr,
> > > -				     int slots);
> > > +int __must_check
> > > +drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state *state,
> > > +			      struct drm_dp_mst_topology_mgr *mgr,
> > > +			      struct drm_dp_mst_port *port, int pbn);
> > > +int __must_check
> > > +drm_dp_atomic_release_vcpi_slots(struct drm_atomic_state *state,
> > > +				 struct drm_dp_mst_topology_mgr *mgr,
> > > +				 struct drm_dp_mst_port *port);
> > >  int drm_dp_send_power_updown_phy(struct drm_dp_mst_topology_mgr *mgr,
> > >  				 struct drm_dp_mst_port *port, bool power_up);
> > > +int __must_check drm_dp_mst_atomic_check(struct drm_atomic_state
> > > *state);
> > >  
> > >  extern const struct drm_private_state_funcs
> > > drm_dp_mst_topology_state_funcs;
> > >  
> > > -- 
> > > 2.19.1
> > > 
> > 
> > -- 
> > Daniel Vetter
> > Software Engineer, Intel Corporation
> > http://blog.ffwll.ch

Daniel Vetter Nov. 27, 2018, 7:44 p.m. UTC | #6

On Tue, Nov 27, 2018 at 12:48:59PM -0500, Lyude Paul wrote:
> On Mon, 2018-11-26 at 22:22 +0100, Daniel Vetter wrote:
> > On Mon, Nov 26, 2018 at 10:04:21PM +0100, Daniel Vetter wrote:
> > > On Thu, Nov 15, 2018 at 07:50:05PM -0500, Lyude Paul wrote:
> > > > There has been a TODO waiting for quite a long time in
> > > > drm_dp_mst_topology.c:
> > > > 
> > > > 	/* We cannot rely on port->vcpi.num_slots to update
> > > > 	 * topology_state->avail_slots as the port may not exist if the parent
> > > > 	 * branch device was unplugged. This should be fixed by tracking
> > > > 	 * per-port slot allocation in drm_dp_mst_topology_state instead of
> > > > 	 * depending on the caller to tell us how many slots to release.
> > > > 	 */
> > > > 
> > > > That's not the only reason we should fix this: forcing the driver to
> > > > track the VCPI allocations throughout a state's atomic check is
> > > > error prone, because it means that extra care has to be taken with the
> > > > order that drm_dp_atomic_find_vcpi_slots() and
> > > > drm_dp_atomic_release_vcpi_slots() are called in in order to ensure
> > > > idempotency. Currently the only driver actually using these helpers,
> > > > i915, doesn't even do this correctly: multiple ->best_encoder() checks
> > > > with i915's current implementation would not be idempotent and would
> > > > over-allocate VCPI slots, something I learned trying to implement
> > > > fallback retraining in MST.
> > > > 
> > > > So: simplify this whole mess, and teach drm_dp_atomic_find_vcpi_slots()
> > > > and drm_dp_atomic_release_vcpi_slots() to track the VCPI allocations for
> > > > each port. This allows us to ensure idempotency without having to rely
> > > > on the driver as much. Additionally: the driver doesn't need to do any
> > > > kind of VCPI slot tracking anymore if it doesn't need it for it's own
> > > > internal state.
> > > > 
> > > > Additionally; this adds a new drm_dp_mst_atomic_check() helper which
> > > > must be used by atomic drivers to perform validity checks for the new
> > > > VCPI allocations incurred by a state.
> > > > 
> > > > Also: update the documentation and make it more obvious that these
> > > > /must/ be called by /all/ atomic drivers supporting MST.
> > > > 
> > > > Changes since v6:
> > > >  - Keep a kref to all of the ports we have allocations on. This required
> > > >    a good bit of changing to when we call drm_dp_find_vcpi_slots(),
> > > >    mainly that we need to ensure that we only redo VCPI allocations on
> > > >    actual mode or CRTC changes, not crtc_state->active changes.
> > > >    Additionally, we no longer take the registration of the DRM connector
> > > >    for each port into account because so long as we have a kref to the
> > > >    port in the new or previous atomic state, the connector will stay
> > > >    registered.
> > > 
> > > I write an entire pile of small nitpits (still included most of them
> > > below), until I realized this here wont work. Delaying the call to destroy
> > > the connector (well, unregister it really) wreaks the design we've come up
> > > with thus far, resulting in most of my comments here.
> > > 
> > > Instead, all we need to do is delay the kfree(port) at the bottom of
> > > drm_dp_destroy_port(). The vcpi allocation structure _only_ needs the
> > > pointer value to stay valid, as a lookup key. It doesn't care at all about
> > > anything actually stored in there. So the only thing we need to delay is
> > > the kfree. I think the simplest way to achieve this is to add a 2nd kref
> > > (port->kfree_ref or something like that), with on reference held by the
> > > port itself (released in drm_dp_destroy_port), and the other one held
> > > as-needed by the vcpi allocation lists.
> > > 
> > > I think if we go with this design instead of retrofitting a semantic
> > > change of the port lifetime itself, all the complications I complain about
> > > below should disappear.
> > 
> > In the above I meant drm_dp_destroy_port or drm_dp_destroy_connector_work.
> > 
> > Aside: I think creating a kref for the final kfree would also solve a
> > bunch of other issues in a much neater way: In
> > 
> > commit f038c5b99fc1332f558b495d136d4f433ee65caa
> > Author: Lyude Paul <[e-mail]>
> > Date:   Tue Nov 13 17:46:14 2018 -0500
> > 
> >     drm/dp_mst: Skip validating ports during destruction, just ref
> > 
> > we could use that kfree reference to make sure the port pointer is alive.
> > This of course means that drm_dp_update_payload_part1() would also need to
> > use the kfree reference for the vcpi allocations. And probably everywhere
> > else (e.g. in your nouveau series for payload information).
> > 
> > That would give us a very clear separation overall between "port can still
> > be used because it's not yet unplugged" vs. "port only hangs around
> > because a bunch of vcpi allocations and other payload things upstream of
> > the port might still need the port lookup key to free resources". Instead
> > of again sprinkling complicated conditions and magic tricks all over the
> > code to figure out whethe we should allow the get_validate_port to succeed
> > or not, or the modeset to succeed or not.
> 
> mhm, I've been experimenting with this and I think I agree this is definitely
> the right solution to go with. It'll end up decomplicating a lot of this :)

Since it looks like we'll use the kfree_ref quite a bit, wrappers for that
would be useful. Still not sure what to call those, best I could come up
with is dp_port_malloc_get/put. Plus a comment explaining what it does.
It's going to be a bit of wtf-is-this no matter what :-/
-Daniel

> > -Daniel
> > 
> > > Piles of comments below.
> > > 
> > > Cheers, Daniel
> > > 
> > > >  - Use the small changes to drm_dp_put_port() to add even more error
> > > >    checking to make misusage of the helpers more obvious. I added this
> > > >    after having to chase down various use-after-free conditions that
> > > >    started popping up from the new helpers so no one else has to
> > > >    troubleshoot that.
> > > >  - Move some accidental DRM_DEBUG_KMS() calls to DRM_DEBUG_ATOMIC()
> > > >  - Update documentation again, note that find/release() should both not
> > > > be
> > > >    called on the same port in a single atomic check phase (but multiple
> > > >    calls to one or the other is OK)
> > > > 
> > > > Changes since v4:
> > > >  - Don't skip the atomic checks for VCPI allocations if no new VCPI
> > > >    allocations happen in a state. This makes the next change I'm about
> > > >    to list here a lot easier to implement.
> > > >  - Don't ignore VCPI allocations on destroyed ports, instead ensure that
> > > >    when ports are destroyed and still have VCPI allocations in the
> > > >    topology state, the only state changes allowed are releasing said
> > > >    ports' VCPI. This prevents a state with a mix of VCPI allocations
> > > >    from destroyed ports, and allocations from valid ports.
> > > > 
> > > > Changes since v3:
> > > >  - Don't release VCPI allocations in the topology state immediately in
> > > >    drm_dp_atomic_release_vcpi_slots(), instead mark them as 0 and skip
> > > >    over them in drm_dp_mst_duplicate_state(). This makes it so
> > > >    drm_dp_atomic_release_vcpi_slots() is still idempotent while also
> > > >    throwing warnings if the driver messes up it's book keeping and tries
> > > >    to release VCPI slots on a port that doesn't have any pre-existing
> > > >    VCPI allocation - danvet
> > > >  - Change mst_state/state in some debugging messages to "mst state"
> > > > 
> > > > Changes since v2:
> > > >  - Use kmemdup() for duplicating MST state - danvet
> > > >  - Move port validation out of duplicate state callback - danvet
> > > >  - Handle looping through MST topology states in
> > > >    drm_dp_mst_atomic_check() so the driver doesn't have to do it
> > > >  - Fix documentation in drm_dp_atomic_find_vcpi_slots()
> > > >  - Move the atomic check for each individual topology state into it's
> > > >    own function, reduces indenting
> > > >  - Don't consider "stale" MST ports when calculating the bandwidth
> > > >    requirements. This is needed because originally we relied on the
> > > >    state duplication functions to prune any stale ports from the new
> > > >    state, which would prevent us from incorrectly considering their
> > > >    bandwidth requirements alongside legitimate new payloads.
> > > >  - Add function references in drm_dp_atomic_release_vcpi_slots() -
> > > > danvet
> > > >  - Annotate atomic VCPI and atomic check functions with __must_check
> > > >    - danvet
> > > > 
> > > > Changes since v1:
> > > >  - Don't use the now-removed ->atomic_check() for private objects hook,
> > > >    just give drivers a function to call themselves
> > > > 
> > > > Signed-off-by: Lyude Paul <[e-mail]>
> > > > Cc: Daniel Vetter <[e-mail]>
> > > > ---
> > > >  drivers/gpu/drm/drm_dp_mst_topology.c | 294 +++++++++++++++++++++++---
> > > >  drivers/gpu/drm/i915/intel_display.c  |   4 +
> > > >  drivers/gpu/drm/i915/intel_dp_mst.c   |  64 +++---
> > > >  include/drm/drm_dp_mst_helper.h       |  24 ++-
> > > >  4 files changed, 319 insertions(+), 67 deletions(-)
> > > > 
> > > > diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c
> > > > b/drivers/gpu/drm/drm_dp_mst_topology.c
> > > > index 00fbe7a2699d..183d0e832ced 100644
> > > > --- a/drivers/gpu/drm/drm_dp_mst_topology.c
> > > > +++ b/drivers/gpu/drm/drm_dp_mst_topology.c
> > > > @@ -2617,21 +2617,42 @@ static int drm_dp_init_vcpi(struct
> > > > drm_dp_mst_topology_mgr *mgr,
> > > >  }
> > > >  
> > > >  /**
> > > > - * drm_dp_atomic_find_vcpi_slots() - Find and add vcpi slots to the
> > > > state
> > > > + * drm_dp_atomic_find_vcpi_slots() - Find and add VCPI slots to the
> > > > state
> > > >   * @state: global atomic state
> > > >   * @mgr: MST topology manager for the port
> > > >   * @port: port to find vcpi slots for
> > > >   * @pbn: bandwidth required for the mode in PBN
> > > >   *
> > > > - * RETURNS:
> > > > - * Total slots in the atomic state assigned for this port or error
> > > > + * Allocates VCPI slots to @port, replacing any previous VCPI
> > > > allocations it
> > > > + * may have had. Any atomic drivers which support MST must call this
> > > > function
> > > > + * in their &drm_encoder_helper_funcs.atomic_check() callback to change
> > > > the
> > > > + * current VCPI allocation for the new state, but only when
> > > > + * &drm_crtc_state.mode_changed or &drm_crtc_state.connectors_changed
> > > > is set
> > > > + * to ensure compatibility with userspace applications that still use
> > > > the
> > > > + * legacy modesetting UAPI.
> > > > + *
> > > > + * Allocations set by this function are not checked against the
> > > > bandwidth
> > > > + * restraints of @mgr until the driver calls drm_dp_mst_atomic_check().
> > > > + *
> > > > + * Additionally, it is OK to call this function multiple times on the
> > > > same
> > > > + * @port as needed. It is not OK however, to call this function and
> > > > + * drm_dp_atomic_release_vcpi_slots() in the same atomic check phase.
> > > > + *
> > > > + * See also:
> > > > + * drm_dp_atomic_release_vcpi_slots()
> > > > + * drm_dp_mst_atomic_check()
> > > > + *
> > > > + * Returns:
> > > > + * Total slots in the atomic state assigned for this port, or a
> > > > negative error
> > > > + * code if the port no longer exists
> > > >   */
> > > >  int drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state *state,
> > > >  				  struct drm_dp_mst_topology_mgr *mgr,
> > > >  				  struct drm_dp_mst_port *port, int pbn)
> > > >  {
> > > >  	struct drm_dp_mst_topology_state *topology_state;
> > > > -	int req_slots;
> > > > +	struct drm_dp_vcpi_allocation *pos, *vcpi = NULL;
> > > > +	int prev_slots, req_slots, ret;
> > > >  
> > > >  	topology_state = drm_atomic_get_mst_topology_state(state, mgr);
> > > >  	if (IS_ERR(topology_state))
> > > > @@ -2640,20 +2661,60 @@ int drm_dp_atomic_find_vcpi_slots(struct
> > > > drm_atomic_state *state,
> > > >  	port = drm_dp_get_validated_port_ref(mgr, port);
> > > >  	if (port == NULL)
> > > >  		return -EINVAL;
> > > > -	req_slots = DIV_ROUND_UP(pbn, mgr->pbn_div);
> > > > -	DRM_DEBUG_KMS("vcpi slots req=%d, avail=%d\n",
> > > > -			req_slots, topology_state->avail_slots);
> > > >  
> > > > -	if (req_slots > topology_state->avail_slots) {
> > > > -		drm_dp_put_port(port);
> > > > -		return -ENOSPC;
> > > > +	topology_state->vcpi_allocated = true;
> > > > +
> > > > +	/* Find the current allocation for this port, if any */
> > > > +	list_for_each_entry(pos, &topology_state->vcpis, next) {
> > > > +		if (pos->port == port) {
> > > > +			vcpi = pos;
> > > > +			prev_slots = vcpi->vcpi;
> > > > +
> > > > +			/*
> > > > +			 * This should never happen, unless the driver tries
> > > > +			 * releasing and allocating the same VCPI allocation,
> > > > +			 * which is an error
> > > > +			 */
> > > > +			if (WARN_ON(!prev_slots)) {
> > > > +				DRM_ERROR("cannot allocate and release VCPI on
> > > > [MST PORT:%p] in the same state\n",
> > > > +					  port);
> > > > +				return -EINVAL;
> > > > +			}
> > > 
> > > Allowing atomic changes to output state (like disabling it on one crtc,
> > > and then enabling on the other, thus both releasing and re-allocating the
> > > vcpi in one go) is kinda the point of atomic. So definitely can't EINVAL
> > > here.
> > > 
> > > Sounds like we need more testcases in igt?
> > > 
> > > > +
> > > > +			break;
> > > > +		}
> > > >  	}
> > > > +	if (!vcpi)
> > > > +		prev_slots = 0;
> > > >  
> > > > -	topology_state->avail_slots -= req_slots;
> > > > -	DRM_DEBUG_KMS("vcpi slots avail=%d", topology_state->avail_slots);
> > > > +	req_slots = DIV_ROUND_UP(pbn, mgr->pbn_div);
> > > > +
> > > > +	DRM_DEBUG_ATOMIC("[CONNECTOR:%d:%s] [MST PORT:%p] VCPI %d -> %d\n",
> > > > +			 port->connector->base.id, port->connector->name,
> > > > +			 port, prev_slots, req_slots);
> > > > +
> > > > +	/* Add the new allocation to the state */
> > > > +	if (!vcpi) {
> > > > +		vcpi = kzalloc(sizeof(*vcpi), GFP_KERNEL);
> > > > +		if (!vcpi) {
> > > > +			ret = -ENOMEM;
> > > > +			goto out;
> > > > +		}
> > > >  
> > > > +		vcpi->port = drm_dp_get_validated_port_ref(mgr, port);
> > > > +		if (!vcpi->port) {
> > > > +			ret = -EINVAL;
> > > > +			kfree(vcpi);
> > > > +			goto out;
> > > > +		}
> > > > +		list_add(&vcpi->next, &topology_state->vcpis);
> > > > +	}
> > > > +	vcpi->vcpi = req_slots;
> > > > +
> > > > +	ret = req_slots;
> > > > +out:
> > > >  	drm_dp_put_port(port);
> > > > -	return req_slots;
> > > > +	return ret;
> > > >  }
> > > >  EXPORT_SYMBOL(drm_dp_atomic_find_vcpi_slots);
> > > >  
> > > > @@ -2661,31 +2722,66 @@ EXPORT_SYMBOL(drm_dp_atomic_find_vcpi_slots);
> > > >   * drm_dp_atomic_release_vcpi_slots() - Release allocated vcpi slots
> > > >   * @state: global atomic state
> > > >   * @mgr: MST topology manager for the port
> > > > - * @slots: number of vcpi slots to release
> > > > + * @port: The port to release the VCPI slots from
> > > >   *
> > > > - * RETURNS:
> > > > - * 0 if @slots were added back to &drm_dp_mst_topology_state-
> > > > >avail_slots or
> > > > - * negative error code
> > > > + * Releases any VCPI slots that have been allocated to a port in the
> > > > atomic
> > > > + * state. Any atomic drivers which support MST must call this function
> > > > in
> > > > + * their &drm_connector_helper_funcs.atomic_check() callback when the
> > > > + * connector will no longer have VCPI allocated (e.g. because it's CRTC
> > > > was
> > > > + * removed) when it had VCPI allocated in the previous atomic state.
> > > > + *
> > > > + * It is OK to call this even if @port has been removed from the
> > > > system.
> > > > + * Additionally, it is OK to call this function multiple times on the
> > > > same
> > > > + * @port as needed. It is not OK however, to call this function and
> > > > + * drm_dp_atomic_find_vcpi_slots() on the same @port in a single atomic
> > > > check
> > > > + * phase.
> > > > + *
> > > > + * See also:
> > > > + * drm_dp_atomic_find_vcpi_slots()
> > > > + * drm_dp_mst_atomic_check()
> > > > + *
> > > > + * Returns:
> > > > + * 0 if all slots for this port were added back to
> > > > + * &drm_dp_mst_topology_state.avail_slots or negative error code
> > > >   */
> > > >  int drm_dp_atomic_release_vcpi_slots(struct drm_atomic_state *state,
> > > >  				     struct drm_dp_mst_topology_mgr *mgr,
> > > > -				     int slots)
> > > > +				     struct drm_dp_mst_port *port)
> > > >  {
> > > >  	struct drm_dp_mst_topology_state *topology_state;
> > > > +	struct drm_dp_vcpi_allocation *pos;
> > > > +	bool found = false;
> > > >  
> > > >  	topology_state = drm_atomic_get_mst_topology_state(state, mgr);
> > > >  	if (IS_ERR(topology_state))
> > > >  		return PTR_ERR(topology_state);
> > > >  
> > > > -	/* We cannot rely on port->vcpi.num_slots to update
> > > > -	 * topology_state->avail_slots as the port may not exist if the parent
> > > > -	 * branch device was unplugged. This should be fixed by tracking
> > > > -	 * per-port slot allocation in drm_dp_mst_topology_state instead of
> > > > -	 * depending on the caller to tell us how many slots to release.
> > > > -	 */
> > > > -	topology_state->avail_slots += slots;
> > > > -	DRM_DEBUG_KMS("vcpi slots released=%d, avail=%d\n",
> > > > -			slots, topology_state->avail_slots);
> > > > +	list_for_each_entry(pos, &topology_state->vcpis, next) {
> > > > +		if (pos->port == port) {
> > > > +			found = true;
> > > > +			break;
> > > > +		}
> > > > +	}
> > > > +	if (WARN_ON(!found)) {
> > > > +		DRM_ERROR("no VCPI for [MST PORT:%p] found in mst state %p\n",
> > > > +			  port, &topology_state->base);
> > > > +		return -EINVAL;
> > > > +	}
> > > > +
> > > > +	DRM_DEBUG_ATOMIC("[MST PORT:%p] VCPI %d -> 0\n", port, pos->vcpi);
> > > > +	if (pos->vcpi) {
> > > > +		/*
> > > > +		 * This should be safe-as the previous state will still hold a
> > > > +		 * reference to this port until we swap states. If it's not,
> > > > +		 * the driver has a bookkeeping error
> > > > +		 */
> > > > +		if (WARN_ON(drm_dp_put_port(port))) {
> > > 
> > > The usual way to do this (instead of adding a return value to the normal
> > > _put function) is something like
> > > 
> > > 	kref_put(port, function_that_with_just_a_WARNING_DRM_ERROR)
> > > 
> > > avoids the need for patch 2.
> > > 
> > > > +			DRM_ERROR("cannot allocate and release VCPI on [MST
> > > > PORT:%p] in the same state\n",
> > > > +				  port);
> > > > +			return -EINVAL;
> > > > +		}
> > > > +		pos->vcpi = 0;
> > > > +	}
> > > >  
> > > >  	return 0;
> > > >  }
> > > > @@ -3115,15 +3211,45 @@ static void drm_dp_destroy_connector_work(struct
> > > > work_struct *work)
> > > >  static struct drm_private_state *
> > > >  drm_dp_mst_duplicate_state(struct drm_private_obj *obj)
> > > >  {
> > > > -	struct drm_dp_mst_topology_state *state;
> > > > +	struct drm_dp_mst_topology_state *state, *old_state =
> > > > +		to_dp_mst_topology_state(obj->state);
> > > > +	struct drm_dp_vcpi_allocation *pos, *vcpi;
> > > >  
> > > > -	state = kmemdup(obj->state, sizeof(*state), GFP_KERNEL);
> > > > +	state = kmemdup(old_state, sizeof(*state), GFP_KERNEL);
> > > >  	if (!state)
> > > >  		return NULL;
> > > >  
> > > >  	__drm_atomic_helper_private_obj_duplicate_state(obj, &state->base);
> > > >  
> > > > +	INIT_LIST_HEAD(&state->vcpis);
> > > > +	state->vcpi_allocated = false;
> > > > +
> > > > +	list_for_each_entry(pos, &old_state->vcpis, next) {
> > > > +		/* Prune leftover freed VCPI allocations */
> > > > +		if (!pos->vcpi)
> > > > +			continue;
> > > > +
> > > > +		vcpi = kmemdup(pos, sizeof(*vcpi), GFP_KERNEL);
> > > > +		if (!vcpi)
> > > > +			goto fail;
> > > > +
> > > > +		/* Take a plain old kref here and don't validate the port, as
> > > > +		 * it's topology might not even exist anymore
> > > > +		 */
> > > > +		kref_get(&vcpi->port->kref);
> > > > +		list_add(&vcpi->next, &state->vcpis);
> > > > +	}
> > > > +
> > > >  	return &state->base;
> > > > +
> > > > +fail:
> > > > +	list_for_each_entry_safe(pos, vcpi, &state->vcpis, next) {
> > > > +		drm_dp_put_port(pos->port);
> > > > +		kfree(pos);
> > > > +	}
> > > > +	kfree(state);
> > > > +
> > > > +	return NULL;
> > > >  }
> > > >  
> > > >  static void drm_dp_mst_destroy_state(struct drm_private_obj *obj,
> > > > @@ -3131,14 +3257,116 @@ static void drm_dp_mst_destroy_state(struct
> > > > drm_private_obj *obj,
> > > >  {
> > > >  	struct drm_dp_mst_topology_state *mst_state =
> > > >  		to_dp_mst_topology_state(state);
> > > > +	struct drm_dp_vcpi_allocation *pos, *tmp;
> > > > +
> > > > +	list_for_each_entry_safe(pos, tmp, &mst_state->vcpis, next) {
> > > > +		/* We only keep references to ports with non-zero VCPIs */
> > > > +		if (pos->vcpi)
> > > > +			drm_dp_put_port(pos->port);
> > > > +		kfree(pos);
> > > > +	}
> > > >  
> > > >  	kfree(mst_state);
> > > >  }
> > > >  
> > > > -static const struct drm_private_state_funcs mst_state_funcs = {
> > > > +static inline int
> > > > +drm_dp_mst_atomic_check_topology_state(struct drm_dp_mst_topology_mgr
> > > > *mgr,
> > > > +				       struct drm_dp_mst_topology_state
> > > > *mst_state)
> > > > +{
> > > > +	struct drm_dp_vcpi_allocation *vcpi;
> > > > +	struct drm_dp_mst_port *port;
> > > > +	int avail_slots = 63, ret;
> > > > +
> > > > +	/* There's no possible scenario where releasing VCPI or keeping it the
> > > > +	 * same would make the state invalid
> > > > +	 */
> > > > +	if (!mst_state->vcpi_allocated) {
> > > 
> > > Do we need this? If not I'm always in favour of removing code if it's not
> > > needed :-)
> > > 
> > > > +		DRM_DEBUG_ATOMIC("[MST MGR:%p] state %p allocates no VCPI,
> > > > check skipped\n",
> > > > +				 mgr, &mst_state->base);
> > > > +		return 0;
> > > > +	}
> > > > +
> > > > +	list_for_each_entry(vcpi, &mst_state->vcpis, next) {
> > > > +		/* Releasing VCPI is always OK-even if the port is gone */
> > > > +		if (!vcpi->vcpi) {
> > > > +			DRM_DEBUG_ATOMIC("[MST PORT:%p] releases all VCPI
> > > > slots\n",
> > > > +					 vcpi->port);
> > > > +			continue;
> > > > +		}
> > > > +
> > > > +		port = drm_dp_get_validated_port_ref(mgr, vcpi->port);
> > > > +		if (!port) {
> > > > +			DRM_DEBUG_ATOMIC("[MST PORT:%p] is gone but still has
> > > > VCPI, cannot add new VCPI\n",
> > > > +					 vcpi->port);
> > > > +			return -EINVAL;
> > > > +		}
> > > 
> > > For find_vcpi we've already done this. For unplugged vcpi, we can't do
> > > this here I think - otherwise if you enable 2 mst outputs on the same
> > > branch, and unplug both, old non-atomic userspace can never ever again
> > > disable them because it can't disable both. The check (to skip already
> > > deallocated ports) above isn't enough to handle that.
> > > 
> > > > +
> > > > +		DRM_DEBUG_ATOMIC("[MST PORT:%p] requires %d vcpi slots\n",
> > > > +				 vcpi->port, vcpi->vcpi);
> > > > +
> > > > +		avail_slots -= vcpi->vcpi;
> > > > +		if (avail_slots < 0) {
> > > > +			DRM_DEBUG_ATOMIC("[MST PORT:%p] not enough VCPI slots
> > > > in mst state %p (avail=%d)\n",
> > > > +					 vcpi->port, mst_state,
> > > > +					 avail_slots + vcpi->vcpi);
> > > > +			ret = -ENOSPC;
> > > > +			goto port_fail;
> > > > +		}
> > > > +
> > > > +		drm_dp_put_port(port);
> > > > +	}
> > > > +	DRM_DEBUG_ATOMIC("[MST MGR:%p] mst state %p VCPI avail=%d used=%d\n",
> > > > +			 mgr, mst_state, avail_slots,
> > > > +			 63 - avail_slots);
> > > > +
> > > > +	return 0;
> > > > +port_fail:
> > > > +	drm_dp_put_port(port);
> > > > +	return ret;
> > > > +}
> > > > +
> > > > +/**
> > > > + * drm_dp_mst_atomic_check - Check that the new state of an MST
> > > > topology in an
> > > > + * atomic update is valid
> > > > + * @state: Pointer to the new &struct drm_dp_mst_topology_state
> > > > + *
> > > > + * Checks the given topology state for an atomic update to ensure that
> > > > it's
> > > > + * valid. This includes checking whether there's enough bandwidth to
> > > > support
> > > > + * the new VCPI allocations in the atomic update.
> > > > + *
> > > > + * Any atomic drivers supporting DP MST must make sure to call this
> > > > after
> > > > + * checking the rest of their state in their
> > > > + * &drm_mode_config_funcs.atomic_check() callback.
> > > > + *
> > > > + * See also:
> > > > + * drm_dp_atomic_find_vcpi_slots()
> > > > + * drm_dp_atomic_release_vcpi_slots()
> > > > + *
> > > > + * Returns:
> > > > + *
> > > > + * 0 if the new state is valid, negative error code otherwise.
> > > > + */
> > > > +int drm_dp_mst_atomic_check(struct drm_atomic_state *state)
> > > > +{
> > > > +	struct drm_dp_mst_topology_mgr *mgr;
> > > > +	struct drm_dp_mst_topology_state *mst_state;
> > > > +	int i, ret = 0;
> > > > +
> > > > +	for_each_new_mst_mgr_in_state(state, mgr, mst_state, i) {
> > > > +		ret = drm_dp_mst_atomic_check_topology_state(mgr, mst_state);
> > > > +		if (ret)
> > > > +			break;
> > > > +	}
> > > > +
> > > > +	return ret;
> > > > +}
> > > > +EXPORT_SYMBOL(drm_dp_mst_atomic_check);
> > > > +
> > > > +const struct drm_private_state_funcs drm_dp_mst_topology_state_funcs =
> > > > {
> > > >  	.atomic_duplicate_state = drm_dp_mst_duplicate_state,
> > > >  	.atomic_destroy_state = drm_dp_mst_destroy_state,
> > > >  };
> > > > +EXPORT_SYMBOL(drm_dp_mst_topology_state_funcs);
> > > >  
> > > >  /**
> > > >   * drm_atomic_get_mst_topology_state: get MST topology state
> > > > @@ -3216,13 +3444,11 @@ int drm_dp_mst_topology_mgr_init(struct
> > > > drm_dp_mst_topology_mgr *mgr,
> > > >  		return -ENOMEM;
> > > >  
> > > >  	mst_state->mgr = mgr;
> > > > -
> > > > -	/* max. time slots - one slot for MTP header */
> > > > -	mst_state->avail_slots = 63;
> > > > +	INIT_LIST_HEAD(&mst_state->vcpis);
> > > >  
> > > >  	drm_atomic_private_obj_init(&mgr->base,
> > > >  				    &mst_state->base,
> > > > -				    &mst_state_funcs);
> > > > +				    &drm_dp_mst_topology_state_funcs);
> > > >  
> > > >  	return 0;
> > > >  }
> > > > diff --git a/drivers/gpu/drm/i915/intel_display.c
> > > > b/drivers/gpu/drm/i915/intel_display.c
> > > > index 132e978227fb..8e85e04a5948 100644
> > > > --- a/drivers/gpu/drm/i915/intel_display.c
> > > > +++ b/drivers/gpu/drm/i915/intel_display.c
> > > > @@ -12592,6 +12592,10 @@ static int intel_atomic_check(struct drm_device
> > > > *dev,
> > > >  				       "[modeset]" : "[fastset]");
> > > >  	}
> > > >  
> > > > +	ret = drm_dp_mst_atomic_check(state);
> > > > +	if (ret)
> > > > +		return ret;
> > > > +
> > > >  	if (any_ms) {
> > > >  		ret = intel_modeset_checks(state);
> > > >  
> > > > diff --git a/drivers/gpu/drm/i915/intel_dp_mst.c
> > > > b/drivers/gpu/drm/i915/intel_dp_mst.c
> > > > index 4de247ddf05f..be2d1ac7d87d 100644
> > > > --- a/drivers/gpu/drm/i915/intel_dp_mst.c
> > > > +++ b/drivers/gpu/drm/i915/intel_dp_mst.c
> > > > @@ -41,8 +41,13 @@ static bool intel_dp_mst_compute_config(struct
> > > > intel_encoder *encoder,
> > > >  	struct drm_connector *connector = conn_state->connector;
> > > >  	void *port = to_intel_connector(connector)->port;
> > > >  	struct drm_atomic_state *state = pipe_config->base.state;
> > > > +	struct drm_crtc *crtc = pipe_config->base.crtc;
> > > > +	struct drm_crtc_state *old_crtc_state =
> > > > +		drm_atomic_get_old_crtc_state(state, crtc);
> > > > +	struct drm_crtc_state *new_crtc_state = &pipe_config->base;
> > > >  	int bpp;
> > > > -	int lane_count, slots = 0;
> > > > +	int lane_count, slots =
> > > > +		to_intel_crtc_state(old_crtc_state)->dp_m_n.tu;
> > > >  	const struct drm_display_mode *adjusted_mode = &pipe_config-
> > > > >base.adjusted_mode;
> > > >  	int mst_pbn;
> > > >  	bool constant_n = drm_dp_has_quirk(&intel_dp->desc,
> > > > @@ -77,8 +82,12 @@ static bool intel_dp_mst_compute_config(struct
> > > > intel_encoder *encoder,
> > > >  	mst_pbn = drm_dp_calc_pbn_mode(adjusted_mode->crtc_clock, bpp);
> > > >  	pipe_config->pbn = mst_pbn;
> > > >  
> > > > -	/* Zombie connectors can't have VCPI slots */
> > > > -	if (!drm_connector_is_unregistered(connector)) {
> > > > +	/* Only change VCPI allocation on actual mode changes, to prevent us
> > > > +	 * from trying to allocate VCPI to ports that no longer exist when we
> > > > +	 * may just be trying to disable DPMS on them
> > > > +	 */
> > > > +	if (new_crtc_state->mode_changed ||
> > > > +	    new_crtc_state->connectors_changed) {
> > > 
> > > Why exactly is this needed? We only call compute_config on the enocoder
> > > when enabling an output. Not when disabling one. Similar for the atomic
> > > helpers and the various encoder callbacks (you have the same change in the
> > > nouveau code). Did this actually blow up while testing?
> > > 
> > > >  		slots = drm_dp_atomic_find_vcpi_slots(state,
> > > >  						      &intel_dp->mst_mgr,
> > > >  						      port,
> > > > @@ -107,36 +116,39 @@ static bool intel_dp_mst_compute_config(struct
> > > > intel_encoder *encoder,
> > > >  	return true;
> > > >  }
> > > >  
> > > > -static int intel_dp_mst_atomic_check(struct drm_connector *connector,
> > > > -		struct drm_connector_state *new_conn_state)
> > > > +static int
> > > > +intel_dp_mst_atomic_check(struct drm_connector *connector,
> > > > +			  struct drm_connector_state *new_conn_state)
> > > >  {
> > > > +	struct intel_connector *intel_connector =
> > > > +		to_intel_connector(connector);
> > > > +	struct drm_dp_mst_topology_mgr *mgr =
> > > > +		&intel_connector->mst_port->mst_mgr;
> > > > +	struct drm_dp_mst_port *port = intel_connector->port;
> > > >  	struct drm_atomic_state *state = new_conn_state->state;
> > > > -	struct drm_connector_state *old_conn_state;
> > > > -	struct drm_crtc *old_crtc;
> > > > -	struct drm_crtc_state *crtc_state;
> > > > -	int slots, ret = 0;
> > > > +	struct drm_connector_state *old_conn_state =
> > > > +		drm_atomic_get_old_connector_state(state, connector);
> > > > +	struct drm_crtc *old_crtc = old_conn_state->crtc,
> > > > +			*new_crtc = new_conn_state->crtc;
> > > > +	struct drm_crtc_state *old_crtc_state, *new_crtc_state;
> > > >  
> > > > -	old_conn_state = drm_atomic_get_old_connector_state(state, connector);
> > > > -	old_crtc = old_conn_state->crtc;
> > > 
> > > Bikeshed: Reworking all the above hides the real changes quite a bit.
> > > 
> > > >  	if (!old_crtc)
> > > > -		return ret;
> > > > +		return 0;
> > > >  
> > > > -	crtc_state = drm_atomic_get_new_crtc_state(state, old_crtc);
> > > > -	slots = to_intel_crtc_state(crtc_state)->dp_m_n.tu;
> > > > -	if (drm_atomic_crtc_needs_modeset(crtc_state) && slots > 0) {
> > > > -		struct drm_dp_mst_topology_mgr *mgr;
> > > > -		struct drm_encoder *old_encoder;
> > > > +	old_crtc_state = drm_atomic_get_old_crtc_state(state, old_crtc);
> > > > +	if (!old_crtc_state ||
> > > 
> > > !old_crtc_state would be a framework bug. Just remove that check.
> > > 
> > > > +	    !to_intel_crtc_state(old_crtc_state)->dp_m_n.tu)
> > > > +		return 0;
> > > 
> > > Hm, why do we need this? Having a DP port previously enabled with a 0 tu
> > > sounds like a pretty big driver bug.
> > > >  
> > > > -		old_encoder = old_conn_state->best_encoder;
> > > > -		mgr = &enc_to_mst(old_encoder)->primary->dp.mst_mgr;
> > > > +	/* If we switched CRTCs, clear the previous one's allocation */
> > > > +	new_crtc_state = drm_atomic_get_new_crtc_state(state, old_crtc);
> > > > +	if (new_crtc_state->connectors_changed && !new_crtc_state->enable)
> > > > +		to_intel_crtc_state(new_crtc_state)->dp_m_n.tu = 0;
> > > 
> > > No idea why we need to clear this here? Shouldn't we just clear this
> > > unconditionally if we release the allocation.
> > > 
> > > Also naming convention, I'd call this old_crtc_new_state, and maybe
> > > old_crtc_old_state. To make it clear these are the old/new states for
> > > old_crtc. Instead of old/new state for the new crtc.
> > > 
> > > >  
> > > > -		ret = drm_dp_atomic_release_vcpi_slots(state, mgr, slots);
> > > > -		if (ret)
> > > > -			DRM_DEBUG_KMS("failed releasing %d vcpi slots:%d\n",
> > > > slots, ret);
> > > > -		else
> > > > -			to_intel_crtc_state(crtc_state)->dp_m_n.tu = 0;
> > > > -	}
> > > > -	return ret;
> > > > +	if (new_crtc)
> > > > +		return 0;
> > > 
> > > Not sure why we shouldn't release this? If we handle this in helpers, it
> > > would remove piles of special casing code here I think. Plus remove
> > > special code in the hlpers.
> > > 
> > > 
> > > > +
> > > > +	return drm_dp_atomic_release_vcpi_slots(state, mgr, port);
> > > >  }
> > > >  
> > > >  static void intel_mst_disable_dp(struct intel_encoder *encoder,
> > > > diff --git a/include/drm/drm_dp_mst_helper.h
> > > > b/include/drm/drm_dp_mst_helper.h
> > > > index 3faceb66f5cb..2392e12b6bce 100644
> > > > --- a/include/drm/drm_dp_mst_helper.h
> > > > +++ b/include/drm/drm_dp_mst_helper.h
> > > > @@ -406,9 +406,16 @@ struct drm_dp_payload {
> > > >  
> > > >  #define to_dp_mst_topology_state(x) container_of(x, struct
> > > > drm_dp_mst_topology_state, base)
> > > >  
> > > > +struct drm_dp_vcpi_allocation {
> > > 
> > > I think a comment here that this pointer holds a full reference (even past
> > > when the connector is unplugged) would be good. Otherwise the loop in
> > > release_vcpi would be buggy.
> > > 
> > > > +	struct drm_dp_mst_port *port;
> > > > +	int vcpi;
> > > > +	struct list_head next;
> > > > +};
> > > > +
> > > >  struct drm_dp_mst_topology_state {
> > > >  	struct drm_private_state base;
> > > > -	int avail_slots;
> > > > +	struct list_head vcpis;
> > > > +	u8 vcpi_allocated;
> > > >  	struct drm_dp_mst_topology_mgr *mgr;
> > > >  };
> > > >  
> > > > @@ -619,14 +626,17 @@ void drm_dp_mst_topology_mgr_suspend(struct
> > > > drm_dp_mst_topology_mgr *mgr);
> > > >  int drm_dp_mst_topology_mgr_resume(struct drm_dp_mst_topology_mgr
> > > > *mgr);
> > > >  struct drm_dp_mst_topology_state
> > > > *drm_atomic_get_mst_topology_state(struct drm_atomic_state *state,
> > > >  								    struct
> > > > drm_dp_mst_topology_mgr *mgr);
> > > > -int drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state *state,
> > > > -				  struct drm_dp_mst_topology_mgr *mgr,
> > > > -				  struct drm_dp_mst_port *port, int pbn);
> > > > -int drm_dp_atomic_release_vcpi_slots(struct drm_atomic_state *state,
> > > > -				     struct drm_dp_mst_topology_mgr *mgr,
> > > > -				     int slots);
> > > > +int __must_check
> > > > +drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state *state,
> > > > +			      struct drm_dp_mst_topology_mgr *mgr,
> > > > +			      struct drm_dp_mst_port *port, int pbn);
> > > > +int __must_check
> > > > +drm_dp_atomic_release_vcpi_slots(struct drm_atomic_state *state,
> > > > +				 struct drm_dp_mst_topology_mgr *mgr,
> > > > +				 struct drm_dp_mst_port *port);
> > > >  int drm_dp_send_power_updown_phy(struct drm_dp_mst_topology_mgr *mgr,
> > > >  				 struct drm_dp_mst_port *port, bool power_up);
> > > > +int __must_check drm_dp_mst_atomic_check(struct drm_atomic_state
> > > > *state);
> > > >  
> > > >  extern const struct drm_private_state_funcs
> > > > drm_dp_mst_topology_state_funcs;
> > > >  
> > > > -- 
> > > > 2.19.1
> > > > 
> > > 
> > > -- 
> > > Daniel Vetter
> > > Software Engineer, Intel Corporation
> > > http://blog.ffwll.ch
> -- 
> Cheers,
> 	Lyude Paul
>

Lyude Paul Nov. 28, 2018, 1:44 a.m. UTC | #7

On Tue, 2018-11-27 at 20:44 +0100, Daniel Vetter wrote:
> On Tue, Nov 27, 2018 at 12:48:59PM -0500, Lyude Paul wrote:
> > On Mon, 2018-11-26 at 22:22 +0100, Daniel Vetter wrote:
> > > On Mon, Nov 26, 2018 at 10:04:21PM +0100, Daniel Vetter wrote:
> > > > On Thu, Nov 15, 2018 at 07:50:05PM -0500, Lyude Paul wrote:
> > > > > There has been a TODO waiting for quite a long time in
> > > > > drm_dp_mst_topology.c:
> > > > >
> > > > > 	/* We cannot rely on port->vcpi.num_slots to update
> > > > > 	 * topology_state->avail_slots as the port may not exist if
> > > > > the parent
> > > > > 	 * branch device was unplugged. This should be fixed by
> > > > > tracking
> > > > > 	 * per-port slot allocation in drm_dp_mst_topology_state
> > > > > instead of
> > > > > 	 * depending on the caller to tell us how many slots to
> > > > > release.
> > > > > 	 */
> > > > >
> > > > > That's not the only reason we should fix this: forcing the driver to
> > > > > track the VCPI allocations throughout a state's atomic check is
> > > > > error prone, because it means that extra care has to be taken with
> > > > > the
> > > > > order that drm_dp_atomic_find_vcpi_slots() and
> > > > > drm_dp_atomic_release_vcpi_slots() are called in in order to ensure
> > > > > idempotency. Currently the only driver actually using these helpers,
> > > > > i915, doesn't even do this correctly: multiple ->best_encoder()
> > > > > checks
> > > > > with i915's current implementation would not be idempotent and would
> > > > > over-allocate VCPI slots, something I learned trying to implement
> > > > > fallback retraining in MST.
> > > > >
> > > > > So: simplify this whole mess, and teach
> > > > > drm_dp_atomic_find_vcpi_slots()
> > > > > and drm_dp_atomic_release_vcpi_slots() to track the VCPI allocations
> > > > > for
> > > > > each port. This allows us to ensure idempotency without having to
> > > > > rely
> > > > > on the driver as much. Additionally: the driver doesn't need to do
> > > > > any
> > > > > kind of VCPI slot tracking anymore if it doesn't need it for it's
> > > > > own
> > > > > internal state.
> > > > >
> > > > > Additionally; this adds a new drm_dp_mst_atomic_check() helper which
> > > > > must be used by atomic drivers to perform validity checks for the
> > > > > new
> > > > > VCPI allocations incurred by a state.
> > > > >
> > > > > Also: update the documentation and make it more obvious that these
> > > > > /must/ be called by /all/ atomic drivers supporting MST.
> > > > >
> > > > > Changes since v6:
> > > > >  - Keep a kref to all of the ports we have allocations on. This
> > > > > required
> > > > >    a good bit of changing to when we call drm_dp_find_vcpi_slots(),
> > > > >    mainly that we need to ensure that we only redo VCPI allocations
> > > > > on
> > > > >    actual mode or CRTC changes, not crtc_state->active changes.
> > > > >    Additionally, we no longer take the registration of the DRM
> > > > > connector
> > > > >    for each port into account because so long as we have a kref to
> > > > > the
> > > > >    port in the new or previous atomic state, the connector will stay
> > > > >    registered.
> > > >
> > > > I write an entire pile of small nitpits (still included most of them
> > > > below), until I realized this here wont work. Delaying the call to
> > > > destroy
> > > > the connector (well, unregister it really) wreaks the design we've
> > > > come up
> > > > with thus far, resulting in most of my comments here.
> > > >
> > > > Instead, all we need to do is delay the kfree(port) at the bottom of
> > > > drm_dp_destroy_port(). The vcpi allocation structure _only_ needs the
> > > > pointer value to stay valid, as a lookup key. It doesn't care at all
> > > > about
> > > > anything actually stored in there. So the only thing we need to delay
> > > > is
> > > > the kfree. I think the simplest way to achieve this is to add a 2nd
> > > > kref
> > > > (port->kfree_ref or something like that), with on reference held by
> > > > the
> > > > port itself (released in drm_dp_destroy_port), and the other one held
> > > > as-needed by the vcpi allocation lists.
> > > >
> > > > I think if we go with this design instead of retrofitting a semantic
> > > > change of the port lifetime itself, all the complications I complain
> > > > about
> > > > below should disappear.
> > >
> > > In the above I meant drm_dp_destroy_port or
> > > drm_dp_destroy_connector_work.
> > >
> > > Aside: I think creating a kref for the final kfree would also solve a
> > > bunch of other issues in a much neater way: In
> > >
> > > commit f038c5b99fc1332f558b495d136d4f433ee65caa
> > > Author: Lyude Paul <[e-mail]>
> > > Date:   Tue Nov 13 17:46:14 2018 -0500
> > >
> > >     drm/dp_mst: Skip validating ports during destruction, just ref
> > >
> > > we could use that kfree reference to make sure the port pointer is
> > > alive.
> > > This of course means that drm_dp_update_payload_part1() would also need
> > > to
> > > use the kfree reference for the vcpi allocations. And probably
> > > everywhere
> > > else (e.g. in your nouveau series for payload information).
> > >
> > > That would give us a very clear separation overall between "port can
> > > still
> > > be used because it's not yet unplugged" vs. "port only hangs around
> > > because a bunch of vcpi allocations and other payload things upstream of
> > > the port might still need the port lookup key to free resources".
> > > Instead
> > > of again sprinkling complicated conditions and magic tricks all over the
> > > code to figure out whethe we should allow the get_validate_port to
> > > succeed
> > > or not, or the modeset to succeed or not.
> >
> > mhm, I've been experimenting with this and I think I agree this is
> > definitely
> > the right solution to go with. It'll end up decomplicating a lot of this
> > :)
>
> Since it looks like we'll use the kfree_ref quite a bit, wrappers for that
> would be useful. Still not sure what to call those, best I could come up
> with is dp_port_malloc_get/put. Plus a comment explaining what it does.
> It's going to be a bit of wtf-is-this no matter what :-/
> -Daniel

We could do this the other way around so it looks like this maybe

struct kref; /* manages kfree() */
struct topology_kref; /* corresonds to lifetime in topology */

Then only expose functions for the normal kref to drivers, so that any
possible confusion is still limited to drm_dp_mst_topology.c

>
> > > -Daniel
> > >
> > > > Piles of comments below.
> > > >
> > > > Cheers, Daniel
> > > >
> > > > >  - Use the small changes to drm_dp_put_port() to add even more error
> > > > >    checking to make misusage of the helpers more obvious. I added
> > > > > this
> > > > >    after having to chase down various use-after-free conditions that
> > > > >    started popping up from the new helpers so no one else has to
> > > > >    troubleshoot that.
> > > > >  - Move some accidental DRM_DEBUG_KMS() calls to DRM_DEBUG_ATOMIC()
> > > > >  - Update documentation again, note that find/release() should both
> > > > > not
> > > > > be
> > > > >    called on the same port in a single atomic check phase (but
> > > > > multiple
> > > > >    calls to one or the other is OK)
> > > > >
> > > > > Changes since v4:
> > > > >  - Don't skip the atomic checks for VCPI allocations if no new VCPI
> > > > >    allocations happen in a state. This makes the next change I'm
> > > > > about
> > > > >    to list here a lot easier to implement.
> > > > >  - Don't ignore VCPI allocations on destroyed ports, instead ensure
> > > > > that
> > > > >    when ports are destroyed and still have VCPI allocations in the
> > > > >    topology state, the only state changes allowed are releasing said
> > > > >    ports' VCPI. This prevents a state with a mix of VCPI allocations
> > > > >    from destroyed ports, and allocations from valid ports.
> > > > >
> > > > > Changes since v3:
> > > > >  - Don't release VCPI allocations in the topology state immediately
> > > > > in
> > > > >    drm_dp_atomic_release_vcpi_slots(), instead mark them as 0 and
> > > > > skip
> > > > >    over them in drm_dp_mst_duplicate_state(). This makes it so
> > > > >    drm_dp_atomic_release_vcpi_slots() is still idempotent while also
> > > > >    throwing warnings if the driver messes up it's book keeping and
> > > > > tries
> > > > >    to release VCPI slots on a port that doesn't have any pre-
> > > > > existing
> > > > >    VCPI allocation - danvet
> > > > >  - Change mst_state/state in some debugging messages to "mst state"
> > > > >
> > > > > Changes since v2:
> > > > >  - Use kmemdup() for duplicating MST state - danvet
> > > > >  - Move port validation out of duplicate state callback - danvet
> > > > >  - Handle looping through MST topology states in
> > > > >    drm_dp_mst_atomic_check() so the driver doesn't have to do it
> > > > >  - Fix documentation in drm_dp_atomic_find_vcpi_slots()
> > > > >  - Move the atomic check for each individual topology state into
> > > > > it's
> > > > >    own function, reduces indenting
> > > > >  - Don't consider "stale" MST ports when calculating the bandwidth
> > > > >    requirements. This is needed because originally we relied on the
> > > > >    state duplication functions to prune any stale ports from the new
> > > > >    state, which would prevent us from incorrectly considering their
> > > > >    bandwidth requirements alongside legitimate new payloads.
> > > > >  - Add function references in drm_dp_atomic_release_vcpi_slots() -
> > > > > danvet
> > > > >  - Annotate atomic VCPI and atomic check functions with __must_check
> > > > >    - danvet
> > > > >
> > > > > Changes since v1:
> > > > >  - Don't use the now-removed ->atomic_check() for private objects
> > > > > hook,
> > > > >    just give drivers a function to call themselves
> > > > >
> > > > > Signed-off-by: Lyude Paul <[e-mail]>
> > > > > Cc: Daniel Vetter <[e-mail]>
> > > > > ---
> > > > >  drivers/gpu/drm/drm_dp_mst_topology.c | 294
> > > > > +++++++++++++++++++++++---
> > > > >  drivers/gpu/drm/i915/intel_display.c  |   4 +
> > > > >  drivers/gpu/drm/i915/intel_dp_mst.c   |  64 +++---
> > > > >  include/drm/drm_dp_mst_helper.h       |  24 ++-
> > > > >  4 files changed, 319 insertions(+), 67 deletions(-)
> > > > >
> > > > > diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c
> > > > > b/drivers/gpu/drm/drm_dp_mst_topology.c
> > > > > index 00fbe7a2699d..183d0e832ced 100644
> > > > > --- a/drivers/gpu/drm/drm_dp_mst_topology.c
> > > > > +++ b/drivers/gpu/drm/drm_dp_mst_topology.c
> > > > > @@ -2617,21 +2617,42 @@ static int drm_dp_init_vcpi(struct
> > > > > drm_dp_mst_topology_mgr *mgr,
> > > > >  }
> > > > >
> > > > >  /**
> > > > > - * drm_dp_atomic_find_vcpi_slots() - Find and add vcpi slots to the
> > > > > state
> > > > > + * drm_dp_atomic_find_vcpi_slots() - Find and add VCPI slots to the
> > > > > state
> > > > >   * @state: global atomic state
> > > > >   * @mgr: MST topology manager for the port
> > > > >   * @port: port to find vcpi slots for
> > > > >   * @pbn: bandwidth required for the mode in PBN
> > > > >   *
> > > > > - * RETURNS:
> > > > > - * Total slots in the atomic state assigned for this port or error
> > > > > + * Allocates VCPI slots to @port, replacing any previous VCPI
> > > > > allocations it
> > > > > + * may have had. Any atomic drivers which support MST must call
> > > > > this
> > > > > function
> > > > > + * in their &drm_encoder_helper_funcs.atomic_check() callback to
> > > > > change
> > > > > the
> > > > > + * current VCPI allocation for the new state, but only when
> > > > > + * &drm_crtc_state.mode_changed or
> > > > > &drm_crtc_state.connectors_changed
> > > > > is set
> > > > > + * to ensure compatibility with userspace applications that still
> > > > > use
> > > > > the
> > > > > + * legacy modesetting UAPI.
> > > > > + *
> > > > > + * Allocations set by this function are not checked against the
> > > > > bandwidth
> > > > > + * restraints of @mgr until the driver calls
> > > > > drm_dp_mst_atomic_check().
> > > > > + *
> > > > > + * Additionally, it is OK to call this function multiple times on
> > > > > the
> > > > > same
> > > > > + * @port as needed. It is not OK however, to call this function and
> > > > > + * drm_dp_atomic_release_vcpi_slots() in the same atomic check
> > > > > phase.
> > > > > + *
> > > > > + * See also:
> > > > > + * drm_dp_atomic_release_vcpi_slots()
> > > > > + * drm_dp_mst_atomic_check()
> > > > > + *
> > > > > + * Returns:
> > > > > + * Total slots in the atomic state assigned for this port, or a
> > > > > negative error
> > > > > + * code if the port no longer exists
> > > > >   */
> > > > >  int drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state *state,
> > > > >  				  struct drm_dp_mst_topology_mgr *mgr,
> > > > >  				  struct drm_dp_mst_port *port, int
> > > > > pbn)
> > > > >  {
> > > > >  	struct drm_dp_mst_topology_state *topology_state;
> > > > > -	int req_slots;
> > > > > +	struct drm_dp_vcpi_allocation *pos, *vcpi = NULL;
> > > > > +	int prev_slots, req_slots, ret;
> > > > >
> > > > >  	topology_state = drm_atomic_get_mst_topology_state(state,
> > > > > mgr);
> > > > >  	if (IS_ERR(topology_state))
> > > > > @@ -2640,20 +2661,60 @@ int drm_dp_atomic_find_vcpi_slots(struct
> > > > > drm_atomic_state *state,
> > > > >  	port = drm_dp_get_validated_port_ref(mgr, port);
> > > > >  	if (port == NULL)
> > > > >  		return -EINVAL;
> > > > > -	req_slots = DIV_ROUND_UP(pbn, mgr->pbn_div);
> > > > > -	DRM_DEBUG_KMS("vcpi slots req=%d, avail=%d\n",
> > > > > -			req_slots, topology_state->avail_slots);
> > > > >
> > > > > -	if (req_slots > topology_state->avail_slots) {
> > > > > -		drm_dp_put_port(port);
> > > > > -		return -ENOSPC;
> > > > > +	topology_state->vcpi_allocated = true;
> > > > > +
> > > > > +	/* Find the current allocation for this port, if any */
> > > > > +	list_for_each_entry(pos, &topology_state->vcpis, next) {
> > > > > +		if (pos->port == port) {
> > > > > +			vcpi = pos;
> > > > > +			prev_slots = vcpi->vcpi;
> > > > > +
> > > > > +			/*
> > > > > +			 * This should never happen, unless the driver
> > > > > tries
> > > > > +			 * releasing and allocating the same VCPI
> > > > > allocation,
> > > > > +			 * which is an error
> > > > > +			 */
> > > > > +			if (WARN_ON(!prev_slots)) {
> > > > > +				DRM_ERROR("cannot allocate and release
> > > > > VCPI on
> > > > > [MST PORT:%p] in the same state\n",
> > > > > +					  port);
> > > > > +				return -EINVAL;
> > > > > +			}
> > > >
> > > > Allowing atomic changes to output state (like disabling it on one
> > > > crtc,
> > > > and then enabling on the other, thus both releasing and re-allocating
> > > > the
> > > > vcpi in one go) is kinda the point of atomic. So definitely can't
> > > > EINVAL
> > > > here.
> > > >
> > > > Sounds like we need more testcases in igt?
> > > >
> > > > > +
> > > > > +			break;
> > > > > +		}
> > > > >  	}
> > > > > +	if (!vcpi)
> > > > > +		prev_slots = 0;
> > > > >
> > > > > -	topology_state->avail_slots -= req_slots;
> > > > > -	DRM_DEBUG_KMS("vcpi slots avail=%d", topology_state-
> > > > > >avail_slots);
> > > > > +	req_slots = DIV_ROUND_UP(pbn, mgr->pbn_div);
> > > > > +
> > > > > +	DRM_DEBUG_ATOMIC("[CONNECTOR:%d:%s] [MST PORT:%p] VCPI %d ->
> > > > > %d\n",
> > > > > +			 port->connector->base.id, port->connector-
> > > > > >name,
> > > > > +			 port, prev_slots, req_slots);
> > > > > +
> > > > > +	/* Add the new allocation to the state */
> > > > > +	if (!vcpi) {
> > > > > +		vcpi = kzalloc(sizeof(*vcpi), GFP_KERNEL);
> > > > > +		if (!vcpi) {
> > > > > +			ret = -ENOMEM;
> > > > > +			goto out;
> > > > > +		}
> > > > >
> > > > > +		vcpi->port = drm_dp_get_validated_port_ref(mgr, port);
> > > > > +		if (!vcpi->port) {
> > > > > +			ret = -EINVAL;
> > > > > +			kfree(vcpi);
> > > > > +			goto out;
> > > > > +		}
> > > > > +		list_add(&vcpi->next, &topology_state->vcpis);
> > > > > +	}
> > > > > +	vcpi->vcpi = req_slots;
> > > > > +
> > > > > +	ret = req_slots;
> > > > > +out:
> > > > >  	drm_dp_put_port(port);
> > > > > -	return req_slots;
> > > > > +	return ret;
> > > > >  }
> > > > >  EXPORT_SYMBOL(drm_dp_atomic_find_vcpi_slots);
> > > > >
> > > > > @@ -2661,31 +2722,66 @@
> > > > > EXPORT_SYMBOL(drm_dp_atomic_find_vcpi_slots);
> > > > >   * drm_dp_atomic_release_vcpi_slots() - Release allocated vcpi
> > > > > slots
> > > > >   * @state: global atomic state
> > > > >   * @mgr: MST topology manager for the port
> > > > > - * @slots: number of vcpi slots to release
> > > > > + * @port: The port to release the VCPI slots from
> > > > >   *
> > > > > - * RETURNS:
> > > > > - * 0 if @slots were added back to &drm_dp_mst_topology_state-
> > > > > > avail_slots or
> > > > > - * negative error code
> > > > > + * Releases any VCPI slots that have been allocated to a port in
> > > > > the
> > > > > atomic
> > > > > + * state. Any atomic drivers which support MST must call this
> > > > > function
> > > > > in
> > > > > + * their &drm_connector_helper_funcs.atomic_check() callback when
> > > > > the
> > > > > + * connector will no longer have VCPI allocated (e.g. because it's
> > > > > CRTC
> > > > > was
> > > > > + * removed) when it had VCPI allocated in the previous atomic
> > > > > state.
> > > > > + *
> > > > > + * It is OK to call this even if @port has been removed from the
> > > > > system.
> > > > > + * Additionally, it is OK to call this function multiple times on
> > > > > the
> > > > > same
> > > > > + * @port as needed. It is not OK however, to call this function and
> > > > > + * drm_dp_atomic_find_vcpi_slots() on the same @port in a single
> > > > > atomic
> > > > > check
> > > > > + * phase.
> > > > > + *
> > > > > + * See also:
> > > > > + * drm_dp_atomic_find_vcpi_slots()
> > > > > + * drm_dp_mst_atomic_check()
> > > > > + *
> > > > > + * Returns:
> > > > > + * 0 if all slots for this port were added back to
> > > > > + * &drm_dp_mst_topology_state.avail_slots or negative error code
> > > > >   */
> > > > >  int drm_dp_atomic_release_vcpi_slots(struct drm_atomic_state
> > > > > *state,
> > > > >  				     struct drm_dp_mst_topology_mgr
> > > > > *mgr,
> > > > > -				     int slots)
> > > > > +				     struct drm_dp_mst_port *port)
> > > > >  {
> > > > >  	struct drm_dp_mst_topology_state *topology_state;
> > > > > +	struct drm_dp_vcpi_allocation *pos;
> > > > > +	bool found = false;
> > > > >
> > > > >  	topology_state = drm_atomic_get_mst_topology_state(state,
> > > > > mgr);
> > > > >  	if (IS_ERR(topology_state))
> > > > >  		return PTR_ERR(topology_state);
> > > > >
> > > > > -	/* We cannot rely on port->vcpi.num_slots to update
> > > > > -	 * topology_state->avail_slots as the port may not exist if
> > > > > the parent
> > > > > -	 * branch device was unplugged. This should be fixed by
> > > > > tracking
> > > > > -	 * per-port slot allocation in drm_dp_mst_topology_state
> > > > > instead of
> > > > > -	 * depending on the caller to tell us how many slots to
> > > > > release.
> > > > > -	 */
> > > > > -	topology_state->avail_slots += slots;
> > > > > -	DRM_DEBUG_KMS("vcpi slots released=%d, avail=%d\n",
> > > > > -			slots, topology_state->avail_slots);
> > > > > +	list_for_each_entry(pos, &topology_state->vcpis, next) {
> > > > > +		if (pos->port == port) {
> > > > > +			found = true;
> > > > > +			break;
> > > > > +		}
> > > > > +	}
> > > > > +	if (WARN_ON(!found)) {
> > > > > +		DRM_ERROR("no VCPI for [MST PORT:%p] found in mst
> > > > > state %p\n",
> > > > > +			  port, &topology_state->base);
> > > > > +		return -EINVAL;
> > > > > +	}
> > > > > +
> > > > > +	DRM_DEBUG_ATOMIC("[MST PORT:%p] VCPI %d -> 0\n", port, pos-
> > > > > >vcpi);
> > > > > +	if (pos->vcpi) {
> > > > > +		/*
> > > > > +		 * This should be safe-as the previous state will
> > > > > still hold a
> > > > > +		 * reference to this port until we swap states. If
> > > > > it's not,
> > > > > +		 * the driver has a bookkeeping error
> > > > > +		 */
> > > > > +		if (WARN_ON(drm_dp_put_port(port))) {
> > > >
> > > > The usual way to do this (instead of adding a return value to the
> > > > normal
> > > > _put function) is something like
> > > >
> > > > 	kref_put(port, function_that_with_just_a_WARNING_DRM_ERROR)
> > > >
> > > > avoids the need for patch 2.
> > > >
> > > > > +			DRM_ERROR("cannot allocate and release VCPI on
> > > > > [MST
> > > > > PORT:%p] in the same state\n",
> > > > > +				  port);
> > > > > +			return -EINVAL;
> > > > > +		}
> > > > > +		pos->vcpi = 0;
> > > > > +	}
> > > > >
> > > > >  	return 0;
> > > > >  }
> > > > > @@ -3115,15 +3211,45 @@ static void
> > > > > drm_dp_destroy_connector_work(struct
> > > > > work_struct *work)
> > > > >  static struct drm_private_state *
> > > > >  drm_dp_mst_duplicate_state(struct drm_private_obj *obj)
> > > > >  {
> > > > > -	struct drm_dp_mst_topology_state *state;
> > > > > +	struct drm_dp_mst_topology_state *state, *old_state =
> > > > > +		to_dp_mst_topology_state(obj->state);
> > > > > +	struct drm_dp_vcpi_allocation *pos, *vcpi;
> > > > >
> > > > > -	state = kmemdup(obj->state, sizeof(*state), GFP_KERNEL);
> > > > > +	state = kmemdup(old_state, sizeof(*state), GFP_KERNEL);
> > > > >  	if (!state)
> > > > >  		return NULL;
> > > > >
> > > > >  	__drm_atomic_helper_private_obj_duplicate_state(obj, &state-
> > > > > >base);
> > > > >
> > > > > +	INIT_LIST_HEAD(&state->vcpis);
> > > > > +	state->vcpi_allocated = false;
> > > > > +
> > > > > +	list_for_each_entry(pos, &old_state->vcpis, next) {
> > > > > +		/* Prune leftover freed VCPI allocations */
> > > > > +		if (!pos->vcpi)
> > > > > +			continue;
> > > > > +
> > > > > +		vcpi = kmemdup(pos, sizeof(*vcpi), GFP_KERNEL);
> > > > > +		if (!vcpi)
> > > > > +			goto fail;
> > > > > +
> > > > > +		/* Take a plain old kref here and don't validate the
> > > > > port, as
> > > > > +		 * it's topology might not even exist anymore
> > > > > +		 */
> > > > > +		kref_get(&vcpi->port->kref);
> > > > > +		list_add(&vcpi->next, &state->vcpis);
> > > > > +	}
> > > > > +
> > > > >  	return &state->base;
> > > > > +
> > > > > +fail:
> > > > > +	list_for_each_entry_safe(pos, vcpi, &state->vcpis, next) {
> > > > > +		drm_dp_put_port(pos->port);
> > > > > +		kfree(pos);
> > > > > +	}
> > > > > +	kfree(state);
> > > > > +
> > > > > +	return NULL;
> > > > >  }
> > > > >
> > > > >  static void drm_dp_mst_destroy_state(struct drm_private_obj *obj,
> > > > > @@ -3131,14 +3257,116 @@ static void drm_dp_mst_destroy_state(struct
> > > > > drm_private_obj *obj,
> > > > >  {
> > > > >  	struct drm_dp_mst_topology_state *mst_state =
> > > > >  		to_dp_mst_topology_state(state);
> > > > > +	struct drm_dp_vcpi_allocation *pos, *tmp;
> > > > > +
> > > > > +	list_for_each_entry_safe(pos, tmp, &mst_state->vcpis, next) {
> > > > > +		/* We only keep references to ports with non-zero
> > > > > VCPIs */
> > > > > +		if (pos->vcpi)
> > > > > +			drm_dp_put_port(pos->port);
> > > > > +		kfree(pos);
> > > > > +	}
> > > > >
> > > > >  	kfree(mst_state);
> > > > >  }
> > > > >
> > > > > -static const struct drm_private_state_funcs mst_state_funcs = {
> > > > > +static inline int
> > > > > +drm_dp_mst_atomic_check_topology_state(struct
> > > > > drm_dp_mst_topology_mgr
> > > > > *mgr,
> > > > > +				       struct
> > > > > drm_dp_mst_topology_state
> > > > > *mst_state)
> > > > > +{
> > > > > +	struct drm_dp_vcpi_allocation *vcpi;
> > > > > +	struct drm_dp_mst_port *port;
> > > > > +	int avail_slots = 63, ret;
> > > > > +
> > > > > +	/* There's no possible scenario where releasing VCPI or
> > > > > keeping it the
> > > > > +	 * same would make the state invalid
> > > > > +	 */
> > > > > +	if (!mst_state->vcpi_allocated) {
> > > >
> > > > Do we need this? If not I'm always in favour of removing code if it's
> > > > not
> > > > needed :-)
> > > >
> > > > > +		DRM_DEBUG_ATOMIC("[MST MGR:%p] state %p allocates no
> > > > > VCPI,
> > > > > check skipped\n",
> > > > > +				 mgr, &mst_state->base);
> > > > > +		return 0;
> > > > > +	}
> > > > > +
> > > > > +	list_for_each_entry(vcpi, &mst_state->vcpis, next) {
> > > > > +		/* Releasing VCPI is always OK-even if the port is
> > > > > gone */
> > > > > +		if (!vcpi->vcpi) {
> > > > > +			DRM_DEBUG_ATOMIC("[MST PORT:%p] releases all
> > > > > VCPI
> > > > > slots\n",
> > > > > +					 vcpi->port);
> > > > > +			continue;
> > > > > +		}
> > > > > +
> > > > > +		port = drm_dp_get_validated_port_ref(mgr, vcpi->port);
> > > > > +		if (!port) {
> > > > > +			DRM_DEBUG_ATOMIC("[MST PORT:%p] is gone but
> > > > > still has
> > > > > VCPI, cannot add new VCPI\n",
> > > > > +					 vcpi->port);
> > > > > +			return -EINVAL;
> > > > > +		}
> > > >
> > > > For find_vcpi we've already done this. For unplugged vcpi, we can't do
> > > > this here I think - otherwise if you enable 2 mst outputs on the same
> > > > branch, and unplug both, old non-atomic userspace can never ever again
> > > > disable them because it can't disable both. The check (to skip already
> > > > deallocated ports) above isn't enough to handle that.
> > > >
> > > > > +
> > > > > +		DRM_DEBUG_ATOMIC("[MST PORT:%p] requires %d vcpi
> > > > > slots\n",
> > > > > +				 vcpi->port, vcpi->vcpi);
> > > > > +
> > > > > +		avail_slots -= vcpi->vcpi;
> > > > > +		if (avail_slots < 0) {
> > > > > +			DRM_DEBUG_ATOMIC("[MST PORT:%p] not enough
> > > > > VCPI slots
> > > > > in mst state %p (avail=%d)\n",
> > > > > +					 vcpi->port, mst_state,
> > > > > +					 avail_slots + vcpi->vcpi);
> > > > > +			ret = -ENOSPC;
> > > > > +			goto port_fail;
> > > > > +		}
> > > > > +
> > > > > +		drm_dp_put_port(port);
> > > > > +	}
> > > > > +	DRM_DEBUG_ATOMIC("[MST MGR:%p] mst state %p VCPI avail=%d
> > > > > used=%d\n",
> > > > > +			 mgr, mst_state, avail_slots,
> > > > > +			 63 - avail_slots);
> > > > > +
> > > > > +	return 0;
> > > > > +port_fail:
> > > > > +	drm_dp_put_port(port);
> > > > > +	return ret;
> > > > > +}
> > > > > +
> > > > > +/**
> > > > > + * drm_dp_mst_atomic_check - Check that the new state of an MST
> > > > > topology in an
> > > > > + * atomic update is valid
> > > > > + * @state: Pointer to the new &struct drm_dp_mst_topology_state
> > > > > + *
> > > > > + * Checks the given topology state for an atomic update to ensure
> > > > > that
> > > > > it's
> > > > > + * valid. This includes checking whether there's enough bandwidth
> > > > > to
> > > > > support
> > > > > + * the new VCPI allocations in the atomic update.
> > > > > + *
> > > > > + * Any atomic drivers supporting DP MST must make sure to call this
> > > > > after
> > > > > + * checking the rest of their state in their
> > > > > + * &drm_mode_config_funcs.atomic_check() callback.
> > > > > + *
> > > > > + * See also:
> > > > > + * drm_dp_atomic_find_vcpi_slots()
> > > > > + * drm_dp_atomic_release_vcpi_slots()
> > > > > + *
> > > > > + * Returns:
> > > > > + *
> > > > > + * 0 if the new state is valid, negative error code otherwise.
> > > > > + */
> > > > > +int drm_dp_mst_atomic_check(struct drm_atomic_state *state)
> > > > > +{
> > > > > +	struct drm_dp_mst_topology_mgr *mgr;
> > > > > +	struct drm_dp_mst_topology_state *mst_state;
> > > > > +	int i, ret = 0;
> > > > > +
> > > > > +	for_each_new_mst_mgr_in_state(state, mgr, mst_state, i) {
> > > > > +		ret = drm_dp_mst_atomic_check_topology_state(mgr,
> > > > > mst_state);
> > > > > +		if (ret)
> > > > > +			break;
> > > > > +	}
> > > > > +
> > > > > +	return ret;
> > > > > +}
> > > > > +EXPORT_SYMBOL(drm_dp_mst_atomic_check);
> > > > > +
> > > > > +const struct drm_private_state_funcs
> > > > > drm_dp_mst_topology_state_funcs =
> > > > > {
> > > > >  	.atomic_duplicate_state = drm_dp_mst_duplicate_state,
> > > > >  	.atomic_destroy_state = drm_dp_mst_destroy_state,
> > > > >  };
> > > > > +EXPORT_SYMBOL(drm_dp_mst_topology_state_funcs);
> > > > >
> > > > >  /**
> > > > >   * drm_atomic_get_mst_topology_state: get MST topology state
> > > > > @@ -3216,13 +3444,11 @@ int drm_dp_mst_topology_mgr_init(struct
> > > > > drm_dp_mst_topology_mgr *mgr,
> > > > >  		return -ENOMEM;
> > > > >
> > > > >  	mst_state->mgr = mgr;
> > > > > -
> > > > > -	/* max. time slots - one slot for MTP header */
> > > > > -	mst_state->avail_slots = 63;
> > > > > +	INIT_LIST_HEAD(&mst_state->vcpis);
> > > > >
> > > > >  	drm_atomic_private_obj_init(&mgr->base,
> > > > >  				    &mst_state->base,
> > > > > -				    &mst_state_funcs);
> > > > > +				    &drm_dp_mst_topology_state_funcs);
> > > > >
> > > > >  	return 0;
> > > > >  }
> > > > > diff --git a/drivers/gpu/drm/i915/intel_display.c
> > > > > b/drivers/gpu/drm/i915/intel_display.c
> > > > > index 132e978227fb..8e85e04a5948 100644
> > > > > --- a/drivers/gpu/drm/i915/intel_display.c
> > > > > +++ b/drivers/gpu/drm/i915/intel_display.c
> > > > > @@ -12592,6 +12592,10 @@ static int intel_atomic_check(struct
> > > > > drm_device
> > > > > *dev,
> > > > >  				       "[modeset]" : "[fastset]");
> > > > >  	}
> > > > >
> > > > > +	ret = drm_dp_mst_atomic_check(state);
> > > > > +	if (ret)
> > > > > +		return ret;
> > > > > +
> > > > >  	if (any_ms) {
> > > > >  		ret = intel_modeset_checks(state);
> > > > >
> > > > > diff --git a/drivers/gpu/drm/i915/intel_dp_mst.c
> > > > > b/drivers/gpu/drm/i915/intel_dp_mst.c
> > > > > index 4de247ddf05f..be2d1ac7d87d 100644
> > > > > --- a/drivers/gpu/drm/i915/intel_dp_mst.c
> > > > > +++ b/drivers/gpu/drm/i915/intel_dp_mst.c
> > > > > @@ -41,8 +41,13 @@ static bool intel_dp_mst_compute_config(struct
> > > > > intel_encoder *encoder,
> > > > >  	struct drm_connector *connector = conn_state->connector;
> > > > >  	void *port = to_intel_connector(connector)->port;
> > > > >  	struct drm_atomic_state *state = pipe_config->base.state;
> > > > > +	struct drm_crtc *crtc = pipe_config->base.crtc;
> > > > > +	struct drm_crtc_state *old_crtc_state =
> > > > > +		drm_atomic_get_old_crtc_state(state, crtc);
> > > > > +	struct drm_crtc_state *new_crtc_state = &pipe_config->base;
> > > > >  	int bpp;
> > > > > -	int lane_count, slots = 0;
> > > > > +	int lane_count, slots =
> > > > > +		to_intel_crtc_state(old_crtc_state)->dp_m_n.tu;
> > > > >  	const struct drm_display_mode *adjusted_mode = &pipe_config-
> > > > > > base.adjusted_mode;
> > > > >  	int mst_pbn;
> > > > >  	bool constant_n = drm_dp_has_quirk(&intel_dp->desc,
> > > > > @@ -77,8 +82,12 @@ static bool intel_dp_mst_compute_config(struct
> > > > > intel_encoder *encoder,
> > > > >  	mst_pbn = drm_dp_calc_pbn_mode(adjusted_mode->crtc_clock,
> > > > > bpp);
> > > > >  	pipe_config->pbn = mst_pbn;
> > > > >
> > > > > -	/* Zombie connectors can't have VCPI slots */
> > > > > -	if (!drm_connector_is_unregistered(connector)) {
> > > > > +	/* Only change VCPI allocation on actual mode changes, to
> > > > > prevent us
> > > > > +	 * from trying to allocate VCPI to ports that no longer exist
> > > > > when we
> > > > > +	 * may just be trying to disable DPMS on them
> > > > > +	 */
> > > > > +	if (new_crtc_state->mode_changed ||
> > > > > +	    new_crtc_state->connectors_changed) {
> > > >
> > > > Why exactly is this needed? We only call compute_config on the
> > > > enocoder
> > > > when enabling an output. Not when disabling one. Similar for the
> > > > atomic
> > > > helpers and the various encoder callbacks (you have the same change in
> > > > the
> > > > nouveau code). Did this actually blow up while testing?
> > > >
> > > > >  		slots = drm_dp_atomic_find_vcpi_slots(state,
> > > > >  						      &intel_dp-
> > > > > >mst_mgr,
> > > > >  						      port,
> > > > > @@ -107,36 +116,39 @@ static bool intel_dp_mst_compute_config(struct
> > > > > intel_encoder *encoder,
> > > > >  	return true;
> > > > >  }
> > > > >
> > > > > -static int intel_dp_mst_atomic_check(struct drm_connector
> > > > > *connector,
> > > > > -		struct drm_connector_state *new_conn_state)
> > > > > +static int
> > > > > +intel_dp_mst_atomic_check(struct drm_connector *connector,
> > > > > +			  struct drm_connector_state *new_conn_state)
> > > > >  {
> > > > > +	struct intel_connector *intel_connector =
> > > > > +		to_intel_connector(connector);
> > > > > +	struct drm_dp_mst_topology_mgr *mgr =
> > > > > +		&intel_connector->mst_port->mst_mgr;
> > > > > +	struct drm_dp_mst_port *port = intel_connector->port;
> > > > >  	struct drm_atomic_state *state = new_conn_state->state;
> > > > > -	struct drm_connector_state *old_conn_state;
> > > > > -	struct drm_crtc *old_crtc;
> > > > > -	struct drm_crtc_state *crtc_state;
> > > > > -	int slots, ret = 0;
> > > > > +	struct drm_connector_state *old_conn_state =
> > > > > +		drm_atomic_get_old_connector_state(state, connector);
> > > > > +	struct drm_crtc *old_crtc = old_conn_state->crtc,
> > > > > +			*new_crtc = new_conn_state->crtc;
> > > > > +	struct drm_crtc_state *old_crtc_state, *new_crtc_state;
> > > > >
> > > > > -	old_conn_state = drm_atomic_get_old_connector_state(state,
> > > > > connector);
> > > > > -	old_crtc = old_conn_state->crtc;
> > > >
> > > > Bikeshed: Reworking all the above hides the real changes quite a bit.
> > > >
> > > > >  	if (!old_crtc)
> > > > > -		return ret;
> > > > > +		return 0;
> > > > >
> > > > > -	crtc_state = drm_atomic_get_new_crtc_state(state, old_crtc);
> > > > > -	slots = to_intel_crtc_state(crtc_state)->dp_m_n.tu;
> > > > > -	if (drm_atomic_crtc_needs_modeset(crtc_state) && slots > 0) {
> > > > > -		struct drm_dp_mst_topology_mgr *mgr;
> > > > > -		struct drm_encoder *old_encoder;
> > > > > +	old_crtc_state = drm_atomic_get_old_crtc_state(state,
> > > > > old_crtc);
> > > > > +	if (!old_crtc_state ||
> > > >
> > > > !old_crtc_state would be a framework bug. Just remove that check.
> > > >
> > > > > +	    !to_intel_crtc_state(old_crtc_state)->dp_m_n.tu)
> > > > > +		return 0;
> > > >
> > > > Hm, why do we need this? Having a DP port previously enabled with a 0
> > > > tu
> > > > sounds like a pretty big driver bug.
> > > > >
> > > > > -		old_encoder = old_conn_state->best_encoder;
> > > > > -		mgr = &enc_to_mst(old_encoder)->primary->dp.mst_mgr;
> > > > > +	/* If we switched CRTCs, clear the previous one's allocation
> > > > > */
> > > > > +	new_crtc_state = drm_atomic_get_new_crtc_state(state,
> > > > > old_crtc);
> > > > > +	if (new_crtc_state->connectors_changed && !new_crtc_state-
> > > > > >enable)
> > > > > +		to_intel_crtc_state(new_crtc_state)->dp_m_n.tu = 0;
> > > >
> > > > No idea why we need to clear this here? Shouldn't we just clear this
> > > > unconditionally if we release the allocation.
> > > >
> > > > Also naming convention, I'd call this old_crtc_new_state, and maybe
> > > > old_crtc_old_state. To make it clear these are the old/new states for
> > > > old_crtc. Instead of old/new state for the new crtc.
> > > >
> > > > >
> > > > > -		ret = drm_dp_atomic_release_vcpi_slots(state, mgr,
> > > > > slots);
> > > > > -		if (ret)
> > > > > -			DRM_DEBUG_KMS("failed releasing %d vcpi
> > > > > slots:%d\n",
> > > > > slots, ret);
> > > > > -		else
> > > > > -			to_intel_crtc_state(crtc_state)->dp_m_n.tu =
> > > > > 0;
> > > > > -	}
> > > > > -	return ret;
> > > > > +	if (new_crtc)
> > > > > +		return 0;
> > > >
> > > > Not sure why we shouldn't release this? If we handle this in helpers,
> > > > it
> > > > would remove piles of special casing code here I think. Plus remove
> > > > special code in the hlpers.
> > > >
> > > >
> > > > > +
> > > > > +	return drm_dp_atomic_release_vcpi_slots(state, mgr, port);
> > > > >  }
> > > > >
> > > > >  static void intel_mst_disable_dp(struct intel_encoder *encoder,
> > > > > diff --git a/include/drm/drm_dp_mst_helper.h
> > > > > b/include/drm/drm_dp_mst_helper.h
> > > > > index 3faceb66f5cb..2392e12b6bce 100644
> > > > > --- a/include/drm/drm_dp_mst_helper.h
> > > > > +++ b/include/drm/drm_dp_mst_helper.h
> > > > > @@ -406,9 +406,16 @@ struct drm_dp_payload {
> > > > >
> > > > >  #define to_dp_mst_topology_state(x) container_of(x, struct
> > > > > drm_dp_mst_topology_state, base)
> > > > >
> > > > > +struct drm_dp_vcpi_allocation {
> > > >
> > > > I think a comment here that this pointer holds a full reference (even
> > > > past
> > > > when the connector is unplugged) would be good. Otherwise the loop in
> > > > release_vcpi would be buggy.
> > > >
> > > > > +	struct drm_dp_mst_port *port;
> > > > > +	int vcpi;
> > > > > +	struct list_head next;
> > > > > +};
> > > > > +
> > > > >  struct drm_dp_mst_topology_state {
> > > > >  	struct drm_private_state base;
> > > > > -	int avail_slots;
> > > > > +	struct list_head vcpis;
> > > > > +	u8 vcpi_allocated;
> > > > >  	struct drm_dp_mst_topology_mgr *mgr;
> > > > >  };
> > > > >
> > > > > @@ -619,14 +626,17 @@ void drm_dp_mst_topology_mgr_suspend(struct
> > > > > drm_dp_mst_topology_mgr *mgr);
> > > > >  int drm_dp_mst_topology_mgr_resume(struct drm_dp_mst_topology_mgr
> > > > > *mgr);
> > > > >  struct drm_dp_mst_topology_state
> > > > > *drm_atomic_get_mst_topology_state(struct drm_atomic_state *state,
> > > > >  								    st
> > > > > ruct
> > > > > drm_dp_mst_topology_mgr *mgr);
> > > > > -int drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state *state,
> > > > > -				  struct drm_dp_mst_topology_mgr *mgr,
> > > > > -				  struct drm_dp_mst_port *port, int
> > > > > pbn);
> > > > > -int drm_dp_atomic_release_vcpi_slots(struct drm_atomic_state
> > > > > *state,
> > > > > -				     struct drm_dp_mst_topology_mgr
> > > > > *mgr,
> > > > > -				     int slots);
> > > > > +int __must_check
> > > > > +drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state *state,
> > > > > +			      struct drm_dp_mst_topology_mgr *mgr,
> > > > > +			      struct drm_dp_mst_port *port, int pbn);
> > > > > +int __must_check
> > > > > +drm_dp_atomic_release_vcpi_slots(struct drm_atomic_state *state,
> > > > > +				 struct drm_dp_mst_topology_mgr *mgr,
> > > > > +				 struct drm_dp_mst_port *port);
> > > > >  int drm_dp_send_power_updown_phy(struct drm_dp_mst_topology_mgr
> > > > > *mgr,
> > > > >  				 struct drm_dp_mst_port *port, bool
> > > > > power_up);
> > > > > +int __must_check drm_dp_mst_atomic_check(struct drm_atomic_state
> > > > > *state);
> > > > >
> > > > >  extern const struct drm_private_state_funcs
> > > > > drm_dp_mst_topology_state_funcs;
> > > > >
> > > > > --
> > > > > 2.19.1
> > > > >
> > > >
> > > > --
> > > > Daniel Vetter
> > > > Software Engineer, Intel Corporation
> > > > http://blog.ffwll.ch
> > --
> > Cheers,
> > 	Lyude Paul
> >
--
Cheers,
	Lyude Paul

Daniel Vetter Nov. 28, 2018, 8:17 a.m. UTC | #8

On Tue, Nov 27, 2018 at 08:44:14PM -0500, Lyude Paul wrote:
> On Tue, 2018-11-27 at 20:44 +0100, Daniel Vetter wrote:
> > On Tue, Nov 27, 2018 at 12:48:59PM -0500, Lyude Paul wrote:
> > > On Mon, 2018-11-26 at 22:22 +0100, Daniel Vetter wrote:
> > > > On Mon, Nov 26, 2018 at 10:04:21PM +0100, Daniel Vetter wrote:
> > > > > On Thu, Nov 15, 2018 at 07:50:05PM -0500, Lyude Paul wrote:
> > > > > > There has been a TODO waiting for quite a long time in
> > > > > > drm_dp_mst_topology.c:
> > > > > >
> > > > > > 	/* We cannot rely on port->vcpi.num_slots to update
> > > > > > 	 * topology_state->avail_slots as the port may not exist if
> > > > > > the parent
> > > > > > 	 * branch device was unplugged. This should be fixed by
> > > > > > tracking
> > > > > > 	 * per-port slot allocation in drm_dp_mst_topology_state
> > > > > > instead of
> > > > > > 	 * depending on the caller to tell us how many slots to
> > > > > > release.
> > > > > > 	 */
> > > > > >
> > > > > > That's not the only reason we should fix this: forcing the driver to
> > > > > > track the VCPI allocations throughout a state's atomic check is
> > > > > > error prone, because it means that extra care has to be taken with
> > > > > > the
> > > > > > order that drm_dp_atomic_find_vcpi_slots() and
> > > > > > drm_dp_atomic_release_vcpi_slots() are called in in order to ensure
> > > > > > idempotency. Currently the only driver actually using these helpers,
> > > > > > i915, doesn't even do this correctly: multiple ->best_encoder()
> > > > > > checks
> > > > > > with i915's current implementation would not be idempotent and would
> > > > > > over-allocate VCPI slots, something I learned trying to implement
> > > > > > fallback retraining in MST.
> > > > > >
> > > > > > So: simplify this whole mess, and teach
> > > > > > drm_dp_atomic_find_vcpi_slots()
> > > > > > and drm_dp_atomic_release_vcpi_slots() to track the VCPI allocations
> > > > > > for
> > > > > > each port. This allows us to ensure idempotency without having to
> > > > > > rely
> > > > > > on the driver as much. Additionally: the driver doesn't need to do
> > > > > > any
> > > > > > kind of VCPI slot tracking anymore if it doesn't need it for it's
> > > > > > own
> > > > > > internal state.
> > > > > >
> > > > > > Additionally; this adds a new drm_dp_mst_atomic_check() helper which
> > > > > > must be used by atomic drivers to perform validity checks for the
> > > > > > new
> > > > > > VCPI allocations incurred by a state.
> > > > > >
> > > > > > Also: update the documentation and make it more obvious that these
> > > > > > /must/ be called by /all/ atomic drivers supporting MST.
> > > > > >
> > > > > > Changes since v6:
> > > > > >  - Keep a kref to all of the ports we have allocations on. This
> > > > > > required
> > > > > >    a good bit of changing to when we call drm_dp_find_vcpi_slots(),
> > > > > >    mainly that we need to ensure that we only redo VCPI allocations
> > > > > > on
> > > > > >    actual mode or CRTC changes, not crtc_state->active changes.
> > > > > >    Additionally, we no longer take the registration of the DRM
> > > > > > connector
> > > > > >    for each port into account because so long as we have a kref to
> > > > > > the
> > > > > >    port in the new or previous atomic state, the connector will stay
> > > > > >    registered.
> > > > >
> > > > > I write an entire pile of small nitpits (still included most of them
> > > > > below), until I realized this here wont work. Delaying the call to
> > > > > destroy
> > > > > the connector (well, unregister it really) wreaks the design we've
> > > > > come up
> > > > > with thus far, resulting in most of my comments here.
> > > > >
> > > > > Instead, all we need to do is delay the kfree(port) at the bottom of
> > > > > drm_dp_destroy_port(). The vcpi allocation structure _only_ needs the
> > > > > pointer value to stay valid, as a lookup key. It doesn't care at all
> > > > > about
> > > > > anything actually stored in there. So the only thing we need to delay
> > > > > is
> > > > > the kfree. I think the simplest way to achieve this is to add a 2nd
> > > > > kref
> > > > > (port->kfree_ref or something like that), with on reference held by
> > > > > the
> > > > > port itself (released in drm_dp_destroy_port), and the other one held
> > > > > as-needed by the vcpi allocation lists.
> > > > >
> > > > > I think if we go with this design instead of retrofitting a semantic
> > > > > change of the port lifetime itself, all the complications I complain
> > > > > about
> > > > > below should disappear.
> > > >
> > > > In the above I meant drm_dp_destroy_port or
> > > > drm_dp_destroy_connector_work.
> > > >
> > > > Aside: I think creating a kref for the final kfree would also solve a
> > > > bunch of other issues in a much neater way: In
> > > >
> > > > commit f038c5b99fc1332f558b495d136d4f433ee65caa
> > > > Author: Lyude Paul <[e-mail]>
> > > > Date:   Tue Nov 13 17:46:14 2018 -0500
> > > >
> > > >     drm/dp_mst: Skip validating ports during destruction, just ref
> > > >
> > > > we could use that kfree reference to make sure the port pointer is
> > > > alive.
> > > > This of course means that drm_dp_update_payload_part1() would also need
> > > > to
> > > > use the kfree reference for the vcpi allocations. And probably
> > > > everywhere
> > > > else (e.g. in your nouveau series for payload information).
> > > >
> > > > That would give us a very clear separation overall between "port can
> > > > still
> > > > be used because it's not yet unplugged" vs. "port only hangs around
> > > > because a bunch of vcpi allocations and other payload things upstream of
> > > > the port might still need the port lookup key to free resources".
> > > > Instead
> > > > of again sprinkling complicated conditions and magic tricks all over the
> > > > code to figure out whethe we should allow the get_validate_port to
> > > > succeed
> > > > or not, or the modeset to succeed or not.
> > >
> > > mhm, I've been experimenting with this and I think I agree this is
> > > definitely
> > > the right solution to go with. It'll end up decomplicating a lot of this
> > > :)
> >
> > Since it looks like we'll use the kfree_ref quite a bit, wrappers for that
> > would be useful. Still not sure what to call those, best I could come up
> > with is dp_port_malloc_get/put. Plus a comment explaining what it does.
> > It's going to be a bit of wtf-is-this no matter what :-/
> > -Daniel
> 
> We could do this the other way around so it looks like this maybe
> 
> struct kref; /* manages kfree() */
> struct topology_kref; /* corresonds to lifetime in topology */
> 
> Then only expose functions for the normal kref to drivers, so that any
> possible confusion is still limited to drm_dp_mst_topology.c

I like this bikeshed a lot. Since we need a bunch of the plain
kref_get/put internally, probably still good to have a wrapper. For that
the port_malloc_get/put() still sounds good to me - port_kfree_get/put
sounds confusing, since it's not the kfree we're refcounting, but the
memory allocation.

Another option would be to add _topology to the public get/put functions,
but that makes for a fairly long function name :-/
-Daniel

> 
> >
> > > > -Daniel
> > > >
> > > > > Piles of comments below.
> > > > >
> > > > > Cheers, Daniel
> > > > >
> > > > > >  - Use the small changes to drm_dp_put_port() to add even more error
> > > > > >    checking to make misusage of the helpers more obvious. I added
> > > > > > this
> > > > > >    after having to chase down various use-after-free conditions that
> > > > > >    started popping up from the new helpers so no one else has to
> > > > > >    troubleshoot that.
> > > > > >  - Move some accidental DRM_DEBUG_KMS() calls to DRM_DEBUG_ATOMIC()
> > > > > >  - Update documentation again, note that find/release() should both
> > > > > > not
> > > > > > be
> > > > > >    called on the same port in a single atomic check phase (but
> > > > > > multiple
> > > > > >    calls to one or the other is OK)
> > > > > >
> > > > > > Changes since v4:
> > > > > >  - Don't skip the atomic checks for VCPI allocations if no new VCPI
> > > > > >    allocations happen in a state. This makes the next change I'm
> > > > > > about
> > > > > >    to list here a lot easier to implement.
> > > > > >  - Don't ignore VCPI allocations on destroyed ports, instead ensure
> > > > > > that
> > > > > >    when ports are destroyed and still have VCPI allocations in the
> > > > > >    topology state, the only state changes allowed are releasing said
> > > > > >    ports' VCPI. This prevents a state with a mix of VCPI allocations
> > > > > >    from destroyed ports, and allocations from valid ports.
> > > > > >
> > > > > > Changes since v3:
> > > > > >  - Don't release VCPI allocations in the topology state immediately
> > > > > > in
> > > > > >    drm_dp_atomic_release_vcpi_slots(), instead mark them as 0 and
> > > > > > skip
> > > > > >    over them in drm_dp_mst_duplicate_state(). This makes it so
> > > > > >    drm_dp_atomic_release_vcpi_slots() is still idempotent while also
> > > > > >    throwing warnings if the driver messes up it's book keeping and
> > > > > > tries
> > > > > >    to release VCPI slots on a port that doesn't have any pre-
> > > > > > existing
> > > > > >    VCPI allocation - danvet
> > > > > >  - Change mst_state/state in some debugging messages to "mst state"
> > > > > >
> > > > > > Changes since v2:
> > > > > >  - Use kmemdup() for duplicating MST state - danvet
> > > > > >  - Move port validation out of duplicate state callback - danvet
> > > > > >  - Handle looping through MST topology states in
> > > > > >    drm_dp_mst_atomic_check() so the driver doesn't have to do it
> > > > > >  - Fix documentation in drm_dp_atomic_find_vcpi_slots()
> > > > > >  - Move the atomic check for each individual topology state into
> > > > > > it's
> > > > > >    own function, reduces indenting
> > > > > >  - Don't consider "stale" MST ports when calculating the bandwidth
> > > > > >    requirements. This is needed because originally we relied on the
> > > > > >    state duplication functions to prune any stale ports from the new
> > > > > >    state, which would prevent us from incorrectly considering their
> > > > > >    bandwidth requirements alongside legitimate new payloads.
> > > > > >  - Add function references in drm_dp_atomic_release_vcpi_slots() -
> > > > > > danvet
> > > > > >  - Annotate atomic VCPI and atomic check functions with __must_check
> > > > > >    - danvet
> > > > > >
> > > > > > Changes since v1:
> > > > > >  - Don't use the now-removed ->atomic_check() for private objects
> > > > > > hook,
> > > > > >    just give drivers a function to call themselves
> > > > > >
> > > > > > Signed-off-by: Lyude Paul <[e-mail]>
> > > > > > Cc: Daniel Vetter <[e-mail]>
> > > > > > ---
> > > > > >  drivers/gpu/drm/drm_dp_mst_topology.c | 294
> > > > > > +++++++++++++++++++++++---
> > > > > >  drivers/gpu/drm/i915/intel_display.c  |   4 +
> > > > > >  drivers/gpu/drm/i915/intel_dp_mst.c   |  64 +++---
> > > > > >  include/drm/drm_dp_mst_helper.h       |  24 ++-
> > > > > >  4 files changed, 319 insertions(+), 67 deletions(-)
> > > > > >
> > > > > > diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c
> > > > > > b/drivers/gpu/drm/drm_dp_mst_topology.c
> > > > > > index 00fbe7a2699d..183d0e832ced 100644
> > > > > > --- a/drivers/gpu/drm/drm_dp_mst_topology.c
> > > > > > +++ b/drivers/gpu/drm/drm_dp_mst_topology.c
> > > > > > @@ -2617,21 +2617,42 @@ static int drm_dp_init_vcpi(struct
> > > > > > drm_dp_mst_topology_mgr *mgr,
> > > > > >  }
> > > > > >
> > > > > >  /**
> > > > > > - * drm_dp_atomic_find_vcpi_slots() - Find and add vcpi slots to the
> > > > > > state
> > > > > > + * drm_dp_atomic_find_vcpi_slots() - Find and add VCPI slots to the
> > > > > > state
> > > > > >   * @state: global atomic state
> > > > > >   * @mgr: MST topology manager for the port
> > > > > >   * @port: port to find vcpi slots for
> > > > > >   * @pbn: bandwidth required for the mode in PBN
> > > > > >   *
> > > > > > - * RETURNS:
> > > > > > - * Total slots in the atomic state assigned for this port or error
> > > > > > + * Allocates VCPI slots to @port, replacing any previous VCPI
> > > > > > allocations it
> > > > > > + * may have had. Any atomic drivers which support MST must call
> > > > > > this
> > > > > > function
> > > > > > + * in their &drm_encoder_helper_funcs.atomic_check() callback to
> > > > > > change
> > > > > > the
> > > > > > + * current VCPI allocation for the new state, but only when
> > > > > > + * &drm_crtc_state.mode_changed or
> > > > > > &drm_crtc_state.connectors_changed
> > > > > > is set
> > > > > > + * to ensure compatibility with userspace applications that still
> > > > > > use
> > > > > > the
> > > > > > + * legacy modesetting UAPI.
> > > > > > + *
> > > > > > + * Allocations set by this function are not checked against the
> > > > > > bandwidth
> > > > > > + * restraints of @mgr until the driver calls
> > > > > > drm_dp_mst_atomic_check().
> > > > > > + *
> > > > > > + * Additionally, it is OK to call this function multiple times on
> > > > > > the
> > > > > > same
> > > > > > + * @port as needed. It is not OK however, to call this function and
> > > > > > + * drm_dp_atomic_release_vcpi_slots() in the same atomic check
> > > > > > phase.
> > > > > > + *
> > > > > > + * See also:
> > > > > > + * drm_dp_atomic_release_vcpi_slots()
> > > > > > + * drm_dp_mst_atomic_check()
> > > > > > + *
> > > > > > + * Returns:
> > > > > > + * Total slots in the atomic state assigned for this port, or a
> > > > > > negative error
> > > > > > + * code if the port no longer exists
> > > > > >   */
> > > > > >  int drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state *state,
> > > > > >  				  struct drm_dp_mst_topology_mgr *mgr,
> > > > > >  				  struct drm_dp_mst_port *port, int
> > > > > > pbn)
> > > > > >  {
> > > > > >  	struct drm_dp_mst_topology_state *topology_state;
> > > > > > -	int req_slots;
> > > > > > +	struct drm_dp_vcpi_allocation *pos, *vcpi = NULL;
> > > > > > +	int prev_slots, req_slots, ret;
> > > > > >
> > > > > >  	topology_state = drm_atomic_get_mst_topology_state(state,
> > > > > > mgr);
> > > > > >  	if (IS_ERR(topology_state))
> > > > > > @@ -2640,20 +2661,60 @@ int drm_dp_atomic_find_vcpi_slots(struct
> > > > > > drm_atomic_state *state,
> > > > > >  	port = drm_dp_get_validated_port_ref(mgr, port);
> > > > > >  	if (port == NULL)
> > > > > >  		return -EINVAL;
> > > > > > -	req_slots = DIV_ROUND_UP(pbn, mgr->pbn_div);
> > > > > > -	DRM_DEBUG_KMS("vcpi slots req=%d, avail=%d\n",
> > > > > > -			req_slots, topology_state->avail_slots);
> > > > > >
> > > > > > -	if (req_slots > topology_state->avail_slots) {
> > > > > > -		drm_dp_put_port(port);
> > > > > > -		return -ENOSPC;
> > > > > > +	topology_state->vcpi_allocated = true;
> > > > > > +
> > > > > > +	/* Find the current allocation for this port, if any */
> > > > > > +	list_for_each_entry(pos, &topology_state->vcpis, next) {
> > > > > > +		if (pos->port == port) {
> > > > > > +			vcpi = pos;
> > > > > > +			prev_slots = vcpi->vcpi;
> > > > > > +
> > > > > > +			/*
> > > > > > +			 * This should never happen, unless the driver
> > > > > > tries
> > > > > > +			 * releasing and allocating the same VCPI
> > > > > > allocation,
> > > > > > +			 * which is an error
> > > > > > +			 */
> > > > > > +			if (WARN_ON(!prev_slots)) {
> > > > > > +				DRM_ERROR("cannot allocate and release
> > > > > > VCPI on
> > > > > > [MST PORT:%p] in the same state\n",
> > > > > > +					  port);
> > > > > > +				return -EINVAL;
> > > > > > +			}
> > > > >
> > > > > Allowing atomic changes to output state (like disabling it on one
> > > > > crtc,
> > > > > and then enabling on the other, thus both releasing and re-allocating
> > > > > the
> > > > > vcpi in one go) is kinda the point of atomic. So definitely can't
> > > > > EINVAL
> > > > > here.
> > > > >
> > > > > Sounds like we need more testcases in igt?
> > > > >
> > > > > > +
> > > > > > +			break;
> > > > > > +		}
> > > > > >  	}
> > > > > > +	if (!vcpi)
> > > > > > +		prev_slots = 0;
> > > > > >
> > > > > > -	topology_state->avail_slots -= req_slots;
> > > > > > -	DRM_DEBUG_KMS("vcpi slots avail=%d", topology_state-
> > > > > > >avail_slots);
> > > > > > +	req_slots = DIV_ROUND_UP(pbn, mgr->pbn_div);
> > > > > > +
> > > > > > +	DRM_DEBUG_ATOMIC("[CONNECTOR:%d:%s] [MST PORT:%p] VCPI %d ->
> > > > > > %d\n",
> > > > > > +			 port->connector->base.id, port->connector-
> > > > > > >name,
> > > > > > +			 port, prev_slots, req_slots);
> > > > > > +
> > > > > > +	/* Add the new allocation to the state */
> > > > > > +	if (!vcpi) {
> > > > > > +		vcpi = kzalloc(sizeof(*vcpi), GFP_KERNEL);
> > > > > > +		if (!vcpi) {
> > > > > > +			ret = -ENOMEM;
> > > > > > +			goto out;
> > > > > > +		}
> > > > > >
> > > > > > +		vcpi->port = drm_dp_get_validated_port_ref(mgr, port);
> > > > > > +		if (!vcpi->port) {
> > > > > > +			ret = -EINVAL;
> > > > > > +			kfree(vcpi);
> > > > > > +			goto out;
> > > > > > +		}
> > > > > > +		list_add(&vcpi->next, &topology_state->vcpis);
> > > > > > +	}
> > > > > > +	vcpi->vcpi = req_slots;
> > > > > > +
> > > > > > +	ret = req_slots;
> > > > > > +out:
> > > > > >  	drm_dp_put_port(port);
> > > > > > -	return req_slots;
> > > > > > +	return ret;
> > > > > >  }
> > > > > >  EXPORT_SYMBOL(drm_dp_atomic_find_vcpi_slots);
> > > > > >
> > > > > > @@ -2661,31 +2722,66 @@
> > > > > > EXPORT_SYMBOL(drm_dp_atomic_find_vcpi_slots);
> > > > > >   * drm_dp_atomic_release_vcpi_slots() - Release allocated vcpi
> > > > > > slots
> > > > > >   * @state: global atomic state
> > > > > >   * @mgr: MST topology manager for the port
> > > > > > - * @slots: number of vcpi slots to release
> > > > > > + * @port: The port to release the VCPI slots from
> > > > > >   *
> > > > > > - * RETURNS:
> > > > > > - * 0 if @slots were added back to &drm_dp_mst_topology_state-
> > > > > > > avail_slots or
> > > > > > - * negative error code
> > > > > > + * Releases any VCPI slots that have been allocated to a port in
> > > > > > the
> > > > > > atomic
> > > > > > + * state. Any atomic drivers which support MST must call this
> > > > > > function
> > > > > > in
> > > > > > + * their &drm_connector_helper_funcs.atomic_check() callback when
> > > > > > the
> > > > > > + * connector will no longer have VCPI allocated (e.g. because it's
> > > > > > CRTC
> > > > > > was
> > > > > > + * removed) when it had VCPI allocated in the previous atomic
> > > > > > state.
> > > > > > + *
> > > > > > + * It is OK to call this even if @port has been removed from the
> > > > > > system.
> > > > > > + * Additionally, it is OK to call this function multiple times on
> > > > > > the
> > > > > > same
> > > > > > + * @port as needed. It is not OK however, to call this function and
> > > > > > + * drm_dp_atomic_find_vcpi_slots() on the same @port in a single
> > > > > > atomic
> > > > > > check
> > > > > > + * phase.
> > > > > > + *
> > > > > > + * See also:
> > > > > > + * drm_dp_atomic_find_vcpi_slots()
> > > > > > + * drm_dp_mst_atomic_check()
> > > > > > + *
> > > > > > + * Returns:
> > > > > > + * 0 if all slots for this port were added back to
> > > > > > + * &drm_dp_mst_topology_state.avail_slots or negative error code
> > > > > >   */
> > > > > >  int drm_dp_atomic_release_vcpi_slots(struct drm_atomic_state
> > > > > > *state,
> > > > > >  				     struct drm_dp_mst_topology_mgr
> > > > > > *mgr,
> > > > > > -				     int slots)
> > > > > > +				     struct drm_dp_mst_port *port)
> > > > > >  {
> > > > > >  	struct drm_dp_mst_topology_state *topology_state;
> > > > > > +	struct drm_dp_vcpi_allocation *pos;
> > > > > > +	bool found = false;
> > > > > >
> > > > > >  	topology_state = drm_atomic_get_mst_topology_state(state,
> > > > > > mgr);
> > > > > >  	if (IS_ERR(topology_state))
> > > > > >  		return PTR_ERR(topology_state);
> > > > > >
> > > > > > -	/* We cannot rely on port->vcpi.num_slots to update
> > > > > > -	 * topology_state->avail_slots as the port may not exist if
> > > > > > the parent
> > > > > > -	 * branch device was unplugged. This should be fixed by
> > > > > > tracking
> > > > > > -	 * per-port slot allocation in drm_dp_mst_topology_state
> > > > > > instead of
> > > > > > -	 * depending on the caller to tell us how many slots to
> > > > > > release.
> > > > > > -	 */
> > > > > > -	topology_state->avail_slots += slots;
> > > > > > -	DRM_DEBUG_KMS("vcpi slots released=%d, avail=%d\n",
> > > > > > -			slots, topology_state->avail_slots);
> > > > > > +	list_for_each_entry(pos, &topology_state->vcpis, next) {
> > > > > > +		if (pos->port == port) {
> > > > > > +			found = true;
> > > > > > +			break;
> > > > > > +		}
> > > > > > +	}
> > > > > > +	if (WARN_ON(!found)) {
> > > > > > +		DRM_ERROR("no VCPI for [MST PORT:%p] found in mst
> > > > > > state %p\n",
> > > > > > +			  port, &topology_state->base);
> > > > > > +		return -EINVAL;
> > > > > > +	}
> > > > > > +
> > > > > > +	DRM_DEBUG_ATOMIC("[MST PORT:%p] VCPI %d -> 0\n", port, pos-
> > > > > > >vcpi);
> > > > > > +	if (pos->vcpi) {
> > > > > > +		/*
> > > > > > +		 * This should be safe-as the previous state will
> > > > > > still hold a
> > > > > > +		 * reference to this port until we swap states. If
> > > > > > it's not,
> > > > > > +		 * the driver has a bookkeeping error
> > > > > > +		 */
> > > > > > +		if (WARN_ON(drm_dp_put_port(port))) {
> > > > >
> > > > > The usual way to do this (instead of adding a return value to the
> > > > > normal
> > > > > _put function) is something like
> > > > >
> > > > > 	kref_put(port, function_that_with_just_a_WARNING_DRM_ERROR)
> > > > >
> > > > > avoids the need for patch 2.
> > > > >
> > > > > > +			DRM_ERROR("cannot allocate and release VCPI on
> > > > > > [MST
> > > > > > PORT:%p] in the same state\n",
> > > > > > +				  port);
> > > > > > +			return -EINVAL;
> > > > > > +		}
> > > > > > +		pos->vcpi = 0;
> > > > > > +	}
> > > > > >
> > > > > >  	return 0;
> > > > > >  }
> > > > > > @@ -3115,15 +3211,45 @@ static void
> > > > > > drm_dp_destroy_connector_work(struct
> > > > > > work_struct *work)
> > > > > >  static struct drm_private_state *
> > > > > >  drm_dp_mst_duplicate_state(struct drm_private_obj *obj)
> > > > > >  {
> > > > > > -	struct drm_dp_mst_topology_state *state;
> > > > > > +	struct drm_dp_mst_topology_state *state, *old_state =
> > > > > > +		to_dp_mst_topology_state(obj->state);
> > > > > > +	struct drm_dp_vcpi_allocation *pos, *vcpi;
> > > > > >
> > > > > > -	state = kmemdup(obj->state, sizeof(*state), GFP_KERNEL);
> > > > > > +	state = kmemdup(old_state, sizeof(*state), GFP_KERNEL);
> > > > > >  	if (!state)
> > > > > >  		return NULL;
> > > > > >
> > > > > >  	__drm_atomic_helper_private_obj_duplicate_state(obj, &state-
> > > > > > >base);
> > > > > >
> > > > > > +	INIT_LIST_HEAD(&state->vcpis);
> > > > > > +	state->vcpi_allocated = false;
> > > > > > +
> > > > > > +	list_for_each_entry(pos, &old_state->vcpis, next) {
> > > > > > +		/* Prune leftover freed VCPI allocations */
> > > > > > +		if (!pos->vcpi)
> > > > > > +			continue;
> > > > > > +
> > > > > > +		vcpi = kmemdup(pos, sizeof(*vcpi), GFP_KERNEL);
> > > > > > +		if (!vcpi)
> > > > > > +			goto fail;
> > > > > > +
> > > > > > +		/* Take a plain old kref here and don't validate the
> > > > > > port, as
> > > > > > +		 * it's topology might not even exist anymore
> > > > > > +		 */
> > > > > > +		kref_get(&vcpi->port->kref);
> > > > > > +		list_add(&vcpi->next, &state->vcpis);
> > > > > > +	}
> > > > > > +
> > > > > >  	return &state->base;
> > > > > > +
> > > > > > +fail:
> > > > > > +	list_for_each_entry_safe(pos, vcpi, &state->vcpis, next) {
> > > > > > +		drm_dp_put_port(pos->port);
> > > > > > +		kfree(pos);
> > > > > > +	}
> > > > > > +	kfree(state);
> > > > > > +
> > > > > > +	return NULL;
> > > > > >  }
> > > > > >
> > > > > >  static void drm_dp_mst_destroy_state(struct drm_private_obj *obj,
> > > > > > @@ -3131,14 +3257,116 @@ static void drm_dp_mst_destroy_state(struct
> > > > > > drm_private_obj *obj,
> > > > > >  {
> > > > > >  	struct drm_dp_mst_topology_state *mst_state =
> > > > > >  		to_dp_mst_topology_state(state);
> > > > > > +	struct drm_dp_vcpi_allocation *pos, *tmp;
> > > > > > +
> > > > > > +	list_for_each_entry_safe(pos, tmp, &mst_state->vcpis, next) {
> > > > > > +		/* We only keep references to ports with non-zero
> > > > > > VCPIs */
> > > > > > +		if (pos->vcpi)
> > > > > > +			drm_dp_put_port(pos->port);
> > > > > > +		kfree(pos);
> > > > > > +	}
> > > > > >
> > > > > >  	kfree(mst_state);
> > > > > >  }
> > > > > >
> > > > > > -static const struct drm_private_state_funcs mst_state_funcs = {
> > > > > > +static inline int
> > > > > > +drm_dp_mst_atomic_check_topology_state(struct
> > > > > > drm_dp_mst_topology_mgr
> > > > > > *mgr,
> > > > > > +				       struct
> > > > > > drm_dp_mst_topology_state
> > > > > > *mst_state)
> > > > > > +{
> > > > > > +	struct drm_dp_vcpi_allocation *vcpi;
> > > > > > +	struct drm_dp_mst_port *port;
> > > > > > +	int avail_slots = 63, ret;
> > > > > > +
> > > > > > +	/* There's no possible scenario where releasing VCPI or
> > > > > > keeping it the
> > > > > > +	 * same would make the state invalid
> > > > > > +	 */
> > > > > > +	if (!mst_state->vcpi_allocated) {
> > > > >
> > > > > Do we need this? If not I'm always in favour of removing code if it's
> > > > > not
> > > > > needed :-)
> > > > >
> > > > > > +		DRM_DEBUG_ATOMIC("[MST MGR:%p] state %p allocates no
> > > > > > VCPI,
> > > > > > check skipped\n",
> > > > > > +				 mgr, &mst_state->base);
> > > > > > +		return 0;
> > > > > > +	}
> > > > > > +
> > > > > > +	list_for_each_entry(vcpi, &mst_state->vcpis, next) {
> > > > > > +		/* Releasing VCPI is always OK-even if the port is
> > > > > > gone */
> > > > > > +		if (!vcpi->vcpi) {
> > > > > > +			DRM_DEBUG_ATOMIC("[MST PORT:%p] releases all
> > > > > > VCPI
> > > > > > slots\n",
> > > > > > +					 vcpi->port);
> > > > > > +			continue;
> > > > > > +		}
> > > > > > +
> > > > > > +		port = drm_dp_get_validated_port_ref(mgr, vcpi->port);
> > > > > > +		if (!port) {
> > > > > > +			DRM_DEBUG_ATOMIC("[MST PORT:%p] is gone but
> > > > > > still has
> > > > > > VCPI, cannot add new VCPI\n",
> > > > > > +					 vcpi->port);
> > > > > > +			return -EINVAL;
> > > > > > +		}
> > > > >
> > > > > For find_vcpi we've already done this. For unplugged vcpi, we can't do
> > > > > this here I think - otherwise if you enable 2 mst outputs on the same
> > > > > branch, and unplug both, old non-atomic userspace can never ever again
> > > > > disable them because it can't disable both. The check (to skip already
> > > > > deallocated ports) above isn't enough to handle that.
> > > > >
> > > > > > +
> > > > > > +		DRM_DEBUG_ATOMIC("[MST PORT:%p] requires %d vcpi
> > > > > > slots\n",
> > > > > > +				 vcpi->port, vcpi->vcpi);
> > > > > > +
> > > > > > +		avail_slots -= vcpi->vcpi;
> > > > > > +		if (avail_slots < 0) {
> > > > > > +			DRM_DEBUG_ATOMIC("[MST PORT:%p] not enough
> > > > > > VCPI slots
> > > > > > in mst state %p (avail=%d)\n",
> > > > > > +					 vcpi->port, mst_state,
> > > > > > +					 avail_slots + vcpi->vcpi);
> > > > > > +			ret = -ENOSPC;
> > > > > > +			goto port_fail;
> > > > > > +		}
> > > > > > +
> > > > > > +		drm_dp_put_port(port);
> > > > > > +	}
> > > > > > +	DRM_DEBUG_ATOMIC("[MST MGR:%p] mst state %p VCPI avail=%d
> > > > > > used=%d\n",
> > > > > > +			 mgr, mst_state, avail_slots,
> > > > > > +			 63 - avail_slots);
> > > > > > +
> > > > > > +	return 0;
> > > > > > +port_fail:
> > > > > > +	drm_dp_put_port(port);
> > > > > > +	return ret;
> > > > > > +}
> > > > > > +
> > > > > > +/**
> > > > > > + * drm_dp_mst_atomic_check - Check that the new state of an MST
> > > > > > topology in an
> > > > > > + * atomic update is valid
> > > > > > + * @state: Pointer to the new &struct drm_dp_mst_topology_state
> > > > > > + *
> > > > > > + * Checks the given topology state for an atomic update to ensure
> > > > > > that
> > > > > > it's
> > > > > > + * valid. This includes checking whether there's enough bandwidth
> > > > > > to
> > > > > > support
> > > > > > + * the new VCPI allocations in the atomic update.
> > > > > > + *
> > > > > > + * Any atomic drivers supporting DP MST must make sure to call this
> > > > > > after
> > > > > > + * checking the rest of their state in their
> > > > > > + * &drm_mode_config_funcs.atomic_check() callback.
> > > > > > + *
> > > > > > + * See also:
> > > > > > + * drm_dp_atomic_find_vcpi_slots()
> > > > > > + * drm_dp_atomic_release_vcpi_slots()
> > > > > > + *
> > > > > > + * Returns:
> > > > > > + *
> > > > > > + * 0 if the new state is valid, negative error code otherwise.
> > > > > > + */
> > > > > > +int drm_dp_mst_atomic_check(struct drm_atomic_state *state)
> > > > > > +{
> > > > > > +	struct drm_dp_mst_topology_mgr *mgr;
> > > > > > +	struct drm_dp_mst_topology_state *mst_state;
> > > > > > +	int i, ret = 0;
> > > > > > +
> > > > > > +	for_each_new_mst_mgr_in_state(state, mgr, mst_state, i) {
> > > > > > +		ret = drm_dp_mst_atomic_check_topology_state(mgr,
> > > > > > mst_state);
> > > > > > +		if (ret)
> > > > > > +			break;
> > > > > > +	}
> > > > > > +
> > > > > > +	return ret;
> > > > > > +}
> > > > > > +EXPORT_SYMBOL(drm_dp_mst_atomic_check);
> > > > > > +
> > > > > > +const struct drm_private_state_funcs
> > > > > > drm_dp_mst_topology_state_funcs =
> > > > > > {
> > > > > >  	.atomic_duplicate_state = drm_dp_mst_duplicate_state,
> > > > > >  	.atomic_destroy_state = drm_dp_mst_destroy_state,
> > > > > >  };
> > > > > > +EXPORT_SYMBOL(drm_dp_mst_topology_state_funcs);
> > > > > >
> > > > > >  /**
> > > > > >   * drm_atomic_get_mst_topology_state: get MST topology state
> > > > > > @@ -3216,13 +3444,11 @@ int drm_dp_mst_topology_mgr_init(struct
> > > > > > drm_dp_mst_topology_mgr *mgr,
> > > > > >  		return -ENOMEM;
> > > > > >
> > > > > >  	mst_state->mgr = mgr;
> > > > > > -
> > > > > > -	/* max. time slots - one slot for MTP header */
> > > > > > -	mst_state->avail_slots = 63;
> > > > > > +	INIT_LIST_HEAD(&mst_state->vcpis);
> > > > > >
> > > > > >  	drm_atomic_private_obj_init(&mgr->base,
> > > > > >  				    &mst_state->base,
> > > > > > -				    &mst_state_funcs);
> > > > > > +				    &drm_dp_mst_topology_state_funcs);
> > > > > >
> > > > > >  	return 0;
> > > > > >  }
> > > > > > diff --git a/drivers/gpu/drm/i915/intel_display.c
> > > > > > b/drivers/gpu/drm/i915/intel_display.c
> > > > > > index 132e978227fb..8e85e04a5948 100644
> > > > > > --- a/drivers/gpu/drm/i915/intel_display.c
> > > > > > +++ b/drivers/gpu/drm/i915/intel_display.c
> > > > > > @@ -12592,6 +12592,10 @@ static int intel_atomic_check(struct
> > > > > > drm_device
> > > > > > *dev,
> > > > > >  				       "[modeset]" : "[fastset]");
> > > > > >  	}
> > > > > >
> > > > > > +	ret = drm_dp_mst_atomic_check(state);
> > > > > > +	if (ret)
> > > > > > +		return ret;
> > > > > > +
> > > > > >  	if (any_ms) {
> > > > > >  		ret = intel_modeset_checks(state);
> > > > > >
> > > > > > diff --git a/drivers/gpu/drm/i915/intel_dp_mst.c
> > > > > > b/drivers/gpu/drm/i915/intel_dp_mst.c
> > > > > > index 4de247ddf05f..be2d1ac7d87d 100644
> > > > > > --- a/drivers/gpu/drm/i915/intel_dp_mst.c
> > > > > > +++ b/drivers/gpu/drm/i915/intel_dp_mst.c
> > > > > > @@ -41,8 +41,13 @@ static bool intel_dp_mst_compute_config(struct
> > > > > > intel_encoder *encoder,
> > > > > >  	struct drm_connector *connector = conn_state->connector;
> > > > > >  	void *port = to_intel_connector(connector)->port;
> > > > > >  	struct drm_atomic_state *state = pipe_config->base.state;
> > > > > > +	struct drm_crtc *crtc = pipe_config->base.crtc;
> > > > > > +	struct drm_crtc_state *old_crtc_state =
> > > > > > +		drm_atomic_get_old_crtc_state(state, crtc);
> > > > > > +	struct drm_crtc_state *new_crtc_state = &pipe_config->base;
> > > > > >  	int bpp;
> > > > > > -	int lane_count, slots = 0;
> > > > > > +	int lane_count, slots =
> > > > > > +		to_intel_crtc_state(old_crtc_state)->dp_m_n.tu;
> > > > > >  	const struct drm_display_mode *adjusted_mode = &pipe_config-
> > > > > > > base.adjusted_mode;
> > > > > >  	int mst_pbn;
> > > > > >  	bool constant_n = drm_dp_has_quirk(&intel_dp->desc,
> > > > > > @@ -77,8 +82,12 @@ static bool intel_dp_mst_compute_config(struct
> > > > > > intel_encoder *encoder,
> > > > > >  	mst_pbn = drm_dp_calc_pbn_mode(adjusted_mode->crtc_clock,
> > > > > > bpp);
> > > > > >  	pipe_config->pbn = mst_pbn;
> > > > > >
> > > > > > -	/* Zombie connectors can't have VCPI slots */
> > > > > > -	if (!drm_connector_is_unregistered(connector)) {
> > > > > > +	/* Only change VCPI allocation on actual mode changes, to
> > > > > > prevent us
> > > > > > +	 * from trying to allocate VCPI to ports that no longer exist
> > > > > > when we
> > > > > > +	 * may just be trying to disable DPMS on them
> > > > > > +	 */
> > > > > > +	if (new_crtc_state->mode_changed ||
> > > > > > +	    new_crtc_state->connectors_changed) {
> > > > >
> > > > > Why exactly is this needed? We only call compute_config on the
> > > > > enocoder
> > > > > when enabling an output. Not when disabling one. Similar for the
> > > > > atomic
> > > > > helpers and the various encoder callbacks (you have the same change in
> > > > > the
> > > > > nouveau code). Did this actually blow up while testing?
> > > > >
> > > > > >  		slots = drm_dp_atomic_find_vcpi_slots(state,
> > > > > >  						      &intel_dp-
> > > > > > >mst_mgr,
> > > > > >  						      port,
> > > > > > @@ -107,36 +116,39 @@ static bool intel_dp_mst_compute_config(struct
> > > > > > intel_encoder *encoder,
> > > > > >  	return true;
> > > > > >  }
> > > > > >
> > > > > > -static int intel_dp_mst_atomic_check(struct drm_connector
> > > > > > *connector,
> > > > > > -		struct drm_connector_state *new_conn_state)
> > > > > > +static int
> > > > > > +intel_dp_mst_atomic_check(struct drm_connector *connector,
> > > > > > +			  struct drm_connector_state *new_conn_state)
> > > > > >  {
> > > > > > +	struct intel_connector *intel_connector =
> > > > > > +		to_intel_connector(connector);
> > > > > > +	struct drm_dp_mst_topology_mgr *mgr =
> > > > > > +		&intel_connector->mst_port->mst_mgr;
> > > > > > +	struct drm_dp_mst_port *port = intel_connector->port;
> > > > > >  	struct drm_atomic_state *state = new_conn_state->state;
> > > > > > -	struct drm_connector_state *old_conn_state;
> > > > > > -	struct drm_crtc *old_crtc;
> > > > > > -	struct drm_crtc_state *crtc_state;
> > > > > > -	int slots, ret = 0;
> > > > > > +	struct drm_connector_state *old_conn_state =
> > > > > > +		drm_atomic_get_old_connector_state(state, connector);
> > > > > > +	struct drm_crtc *old_crtc = old_conn_state->crtc,
> > > > > > +			*new_crtc = new_conn_state->crtc;
> > > > > > +	struct drm_crtc_state *old_crtc_state, *new_crtc_state;
> > > > > >
> > > > > > -	old_conn_state = drm_atomic_get_old_connector_state(state,
> > > > > > connector);
> > > > > > -	old_crtc = old_conn_state->crtc;
> > > > >
> > > > > Bikeshed: Reworking all the above hides the real changes quite a bit.
> > > > >
> > > > > >  	if (!old_crtc)
> > > > > > -		return ret;
> > > > > > +		return 0;
> > > > > >
> > > > > > -	crtc_state = drm_atomic_get_new_crtc_state(state, old_crtc);
> > > > > > -	slots = to_intel_crtc_state(crtc_state)->dp_m_n.tu;
> > > > > > -	if (drm_atomic_crtc_needs_modeset(crtc_state) && slots > 0) {
> > > > > > -		struct drm_dp_mst_topology_mgr *mgr;
> > > > > > -		struct drm_encoder *old_encoder;
> > > > > > +	old_crtc_state = drm_atomic_get_old_crtc_state(state,
> > > > > > old_crtc);
> > > > > > +	if (!old_crtc_state ||
> > > > >
> > > > > !old_crtc_state would be a framework bug. Just remove that check.
> > > > >
> > > > > > +	    !to_intel_crtc_state(old_crtc_state)->dp_m_n.tu)
> > > > > > +		return 0;
> > > > >
> > > > > Hm, why do we need this? Having a DP port previously enabled with a 0
> > > > > tu
> > > > > sounds like a pretty big driver bug.
> > > > > >
> > > > > > -		old_encoder = old_conn_state->best_encoder;
> > > > > > -		mgr = &enc_to_mst(old_encoder)->primary->dp.mst_mgr;
> > > > > > +	/* If we switched CRTCs, clear the previous one's allocation
> > > > > > */
> > > > > > +	new_crtc_state = drm_atomic_get_new_crtc_state(state,
> > > > > > old_crtc);
> > > > > > +	if (new_crtc_state->connectors_changed && !new_crtc_state-
> > > > > > >enable)
> > > > > > +		to_intel_crtc_state(new_crtc_state)->dp_m_n.tu = 0;
> > > > >
> > > > > No idea why we need to clear this here? Shouldn't we just clear this
> > > > > unconditionally if we release the allocation.
> > > > >
> > > > > Also naming convention, I'd call this old_crtc_new_state, and maybe
> > > > > old_crtc_old_state. To make it clear these are the old/new states for
> > > > > old_crtc. Instead of old/new state for the new crtc.
> > > > >
> > > > > >
> > > > > > -		ret = drm_dp_atomic_release_vcpi_slots(state, mgr,
> > > > > > slots);
> > > > > > -		if (ret)
> > > > > > -			DRM_DEBUG_KMS("failed releasing %d vcpi
> > > > > > slots:%d\n",
> > > > > > slots, ret);
> > > > > > -		else
> > > > > > -			to_intel_crtc_state(crtc_state)->dp_m_n.tu =
> > > > > > 0;
> > > > > > -	}
> > > > > > -	return ret;
> > > > > > +	if (new_crtc)
> > > > > > +		return 0;
> > > > >
> > > > > Not sure why we shouldn't release this? If we handle this in helpers,
> > > > > it
> > > > > would remove piles of special casing code here I think. Plus remove
> > > > > special code in the hlpers.
> > > > >
> > > > >
> > > > > > +
> > > > > > +	return drm_dp_atomic_release_vcpi_slots(state, mgr, port);
> > > > > >  }
> > > > > >
> > > > > >  static void intel_mst_disable_dp(struct intel_encoder *encoder,
> > > > > > diff --git a/include/drm/drm_dp_mst_helper.h
> > > > > > b/include/drm/drm_dp_mst_helper.h
> > > > > > index 3faceb66f5cb..2392e12b6bce 100644
> > > > > > --- a/include/drm/drm_dp_mst_helper.h
> > > > > > +++ b/include/drm/drm_dp_mst_helper.h
> > > > > > @@ -406,9 +406,16 @@ struct drm_dp_payload {
> > > > > >
> > > > > >  #define to_dp_mst_topology_state(x) container_of(x, struct
> > > > > > drm_dp_mst_topology_state, base)
> > > > > >
> > > > > > +struct drm_dp_vcpi_allocation {
> > > > >
> > > > > I think a comment here that this pointer holds a full reference (even
> > > > > past
> > > > > when the connector is unplugged) would be good. Otherwise the loop in
> > > > > release_vcpi would be buggy.
> > > > >
> > > > > > +	struct drm_dp_mst_port *port;
> > > > > > +	int vcpi;
> > > > > > +	struct list_head next;
> > > > > > +};
> > > > > > +
> > > > > >  struct drm_dp_mst_topology_state {
> > > > > >  	struct drm_private_state base;
> > > > > > -	int avail_slots;
> > > > > > +	struct list_head vcpis;
> > > > > > +	u8 vcpi_allocated;
> > > > > >  	struct drm_dp_mst_topology_mgr *mgr;
> > > > > >  };
> > > > > >
> > > > > > @@ -619,14 +626,17 @@ void drm_dp_mst_topology_mgr_suspend(struct
> > > > > > drm_dp_mst_topology_mgr *mgr);
> > > > > >  int drm_dp_mst_topology_mgr_resume(struct drm_dp_mst_topology_mgr
> > > > > > *mgr);
> > > > > >  struct drm_dp_mst_topology_state
> > > > > > *drm_atomic_get_mst_topology_state(struct drm_atomic_state *state,
> > > > > >  								    st
> > > > > > ruct
> > > > > > drm_dp_mst_topology_mgr *mgr);
> > > > > > -int drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state *state,
> > > > > > -				  struct drm_dp_mst_topology_mgr *mgr,
> > > > > > -				  struct drm_dp_mst_port *port, int
> > > > > > pbn);
> > > > > > -int drm_dp_atomic_release_vcpi_slots(struct drm_atomic_state
> > > > > > *state,
> > > > > > -				     struct drm_dp_mst_topology_mgr
> > > > > > *mgr,
> > > > > > -				     int slots);
> > > > > > +int __must_check
> > > > > > +drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state *state,
> > > > > > +			      struct drm_dp_mst_topology_mgr *mgr,
> > > > > > +			      struct drm_dp_mst_port *port, int pbn);
> > > > > > +int __must_check
> > > > > > +drm_dp_atomic_release_vcpi_slots(struct drm_atomic_state *state,
> > > > > > +				 struct drm_dp_mst_topology_mgr *mgr,
> > > > > > +				 struct drm_dp_mst_port *port);
> > > > > >  int drm_dp_send_power_updown_phy(struct drm_dp_mst_topology_mgr
> > > > > > *mgr,
> > > > > >  				 struct drm_dp_mst_port *port, bool
> > > > > > power_up);
> > > > > > +int __must_check drm_dp_mst_atomic_check(struct drm_atomic_state
> > > > > > *state);
> > > > > >
> > > > > >  extern const struct drm_private_state_funcs
> > > > > > drm_dp_mst_topology_state_funcs;
> > > > > >
> > > > > > --
> > > > > > 2.19.1
> > > > > >
> > > > >
> > > > > --
> > > > > Daniel Vetter
> > > > > Software Engineer, Intel Corporation
> > > > > http://blog.ffwll.ch
> > > --
> > > Cheers,
> > > 	Lyude Paul
> > >
> --
> Cheers,
> 	Lyude Paul
>

Lyude Paul Nov. 28, 2018, 6:48 p.m. UTC | #9

On Wed, 2018-11-28 at 09:17 +0100, Daniel Vetter wrote:
> On Tue, Nov 27, 2018 at 08:44:14PM -0500, Lyude Paul wrote:
> > On Tue, 2018-11-27 at 20:44 +0100, Daniel Vetter wrote:
> > > On Tue, Nov 27, 2018 at 12:48:59PM -0500, Lyude Paul wrote:
> > > > On Mon, 2018-11-26 at 22:22 +0100, Daniel Vetter wrote:
> > > > > On Mon, Nov 26, 2018 at 10:04:21PM +0100, Daniel Vetter wrote:
> > > > > > On Thu, Nov 15, 2018 at 07:50:05PM -0500, Lyude Paul wrote:
> > > > > > > There has been a TODO waiting for quite a long time in
> > > > > > > drm_dp_mst_topology.c:
> > > > > > > 
> > > > > > > 	/* We cannot rely on port->vcpi.num_slots to update
> > > > > > > 	 * topology_state->avail_slots as the port may not exist if
> > > > > > > the parent
> > > > > > > 	 * branch device was unplugged. This should be fixed by
> > > > > > > tracking
> > > > > > > 	 * per-port slot allocation in drm_dp_mst_topology_state
> > > > > > > instead of
> > > > > > > 	 * depending on the caller to tell us how many slots to
> > > > > > > release.
> > > > > > > 	 */
> > > > > > > 
> > > > > > > That's not the only reason we should fix this: forcing the
> > > > > > > driver to
> > > > > > > track the VCPI allocations throughout a state's atomic check is
> > > > > > > error prone, because it means that extra care has to be taken
> > > > > > > with
> > > > > > > the
> > > > > > > order that drm_dp_atomic_find_vcpi_slots() and
> > > > > > > drm_dp_atomic_release_vcpi_slots() are called in in order to
> > > > > > > ensure
> > > > > > > idempotency. Currently the only driver actually using these
> > > > > > > helpers,
> > > > > > > i915, doesn't even do this correctly: multiple ->best_encoder()
> > > > > > > checks
> > > > > > > with i915's current implementation would not be idempotent and
> > > > > > > would
> > > > > > > over-allocate VCPI slots, something I learned trying to
> > > > > > > implement
> > > > > > > fallback retraining in MST.
> > > > > > > 
> > > > > > > So: simplify this whole mess, and teach
> > > > > > > drm_dp_atomic_find_vcpi_slots()
> > > > > > > and drm_dp_atomic_release_vcpi_slots() to track the VCPI
> > > > > > > allocations
> > > > > > > for
> > > > > > > each port. This allows us to ensure idempotency without having
> > > > > > > to
> > > > > > > rely
> > > > > > > on the driver as much. Additionally: the driver doesn't need to
> > > > > > > do
> > > > > > > any
> > > > > > > kind of VCPI slot tracking anymore if it doesn't need it for
> > > > > > > it's
> > > > > > > own
> > > > > > > internal state.
> > > > > > > 
> > > > > > > Additionally; this adds a new drm_dp_mst_atomic_check() helper
> > > > > > > which
> > > > > > > must be used by atomic drivers to perform validity checks for
> > > > > > > the
> > > > > > > new
> > > > > > > VCPI allocations incurred by a state.
> > > > > > > 
> > > > > > > Also: update the documentation and make it more obvious that
> > > > > > > these
> > > > > > > /must/ be called by /all/ atomic drivers supporting MST.
> > > > > > > 
> > > > > > > Changes since v6:
> > > > > > >  - Keep a kref to all of the ports we have allocations on. This
> > > > > > > required
> > > > > > >    a good bit of changing to when we call
> > > > > > > drm_dp_find_vcpi_slots(),
> > > > > > >    mainly that we need to ensure that we only redo VCPI
> > > > > > > allocations
> > > > > > > on
> > > > > > >    actual mode or CRTC changes, not crtc_state->active changes.
> > > > > > >    Additionally, we no longer take the registration of the DRM
> > > > > > > connector
> > > > > > >    for each port into account because so long as we have a kref
> > > > > > > to
> > > > > > > the
> > > > > > >    port in the new or previous atomic state, the connector will
> > > > > > > stay
> > > > > > >    registered.
> > > > > > 
> > > > > > I write an entire pile of small nitpits (still included most of
> > > > > > them
> > > > > > below), until I realized this here wont work. Delaying the call to
> > > > > > destroy
> > > > > > the connector (well, unregister it really) wreaks the design we've
> > > > > > come up
> > > > > > with thus far, resulting in most of my comments here.
> > > > > > 
> > > > > > Instead, all we need to do is delay the kfree(port) at the bottom
> > > > > > of
> > > > > > drm_dp_destroy_port(). The vcpi allocation structure _only_ needs
> > > > > > the
> > > > > > pointer value to stay valid, as a lookup key. It doesn't care at
> > > > > > all
> > > > > > about
> > > > > > anything actually stored in there. So the only thing we need to
> > > > > > delay
> > > > > > is
> > > > > > the kfree. I think the simplest way to achieve this is to add a
> > > > > > 2nd
> > > > > > kref
> > > > > > (port->kfree_ref or something like that), with on reference held
> > > > > > by
> > > > > > the
> > > > > > port itself (released in drm_dp_destroy_port), and the other one
> > > > > > held
> > > > > > as-needed by the vcpi allocation lists.
> > > > > > 
> > > > > > I think if we go with this design instead of retrofitting a
> > > > > > semantic
> > > > > > change of the port lifetime itself, all the complications I
> > > > > > complain
> > > > > > about
> > > > > > below should disappear.
> > > > > 
> > > > > In the above I meant drm_dp_destroy_port or
> > > > > drm_dp_destroy_connector_work.
> > > > > 
> > > > > Aside: I think creating a kref for the final kfree would also solve
> > > > > a
> > > > > bunch of other issues in a much neater way: In
> > > > > 
> > > > > commit f038c5b99fc1332f558b495d136d4f433ee65caa
> > > > > Author: Lyude Paul <[e-mail]>
> > > > > Date:   Tue Nov 13 17:46:14 2018 -0500
> > > > > 
> > > > >     drm/dp_mst: Skip validating ports during destruction, just ref
> > > > > 
> > > > > we could use that kfree reference to make sure the port pointer is
> > > > > alive.
> > > > > This of course means that drm_dp_update_payload_part1() would also
> > > > > need
> > > > > to
> > > > > use the kfree reference for the vcpi allocations. And probably
> > > > > everywhere
> > > > > else (e.g. in your nouveau series for payload information).
> > > > > 
> > > > > That would give us a very clear separation overall between "port can
> > > > > still
> > > > > be used because it's not yet unplugged" vs. "port only hangs around
> > > > > because a bunch of vcpi allocations and other payload things
> > > > > upstream of
> > > > > the port might still need the port lookup key to free resources".
> > > > > Instead
> > > > > of again sprinkling complicated conditions and magic tricks all over
> > > > > the
> > > > > code to figure out whethe we should allow the get_validate_port to
> > > > > succeed
> > > > > or not, or the modeset to succeed or not.
> > > > 
> > > > mhm, I've been experimenting with this and I think I agree this is
> > > > definitely
> > > > the right solution to go with. It'll end up decomplicating a lot of
> > > > this
> > > > :)
> > > 
> > > Since it looks like we'll use the kfree_ref quite a bit, wrappers for
> > > that
> > > would be useful. Still not sure what to call those, best I could come up
> > > with is dp_port_malloc_get/put. Plus a comment explaining what it does.
> > > It's going to be a bit of wtf-is-this no matter what :-/
> > > -Daniel
> > 
> > We could do this the other way around so it looks like this maybe
> > 
> > struct kref; /* manages kfree() */
> > struct topology_kref; /* corresonds to lifetime in topology */
> > 
> > Then only expose functions for the normal kref to drivers, so that any
> > possible confusion is still limited to drm_dp_mst_topology.c
> 
> I like this bikeshed a lot. Since we need a bunch of the plain
> kref_get/put internally, probably still good to have a wrapper. For that
> the port_malloc_get/put() still sounds good to me - port_kfree_get/put
> sounds confusing, since it's not the kfree we're refcounting, but the
> memory allocation.
> 
> Another option would be to add _topology to the public get/put functions,
> but that makes for a fairly long function name :-/

Well if we exposed get/put functions for this publically, I'd probably just
call them drm_dp_get_port() and drm_dp_put_port() since it wouldn't make sense
to have differentiated naming if we're only exposing one of the krefs
publically (e.g. the malloc kref).

Also: I've been thinking about shortening some of the names of the MST helper
functions for a while. I don't think we need to say drm_dp_mst_topology_mgr
everywhere :)

> -Daniel
> 
> > > > > -Daniel
> > > > > 
> > > > > > Piles of comments below.
> > > > > > 
> > > > > > Cheers, Daniel
> > > > > > 
> > > > > > >  - Use the small changes to drm_dp_put_port() to add even more
> > > > > > > error
> > > > > > >    checking to make misusage of the helpers more obvious. I
> > > > > > > added
> > > > > > > this
> > > > > > >    after having to chase down various use-after-free conditions
> > > > > > > that
> > > > > > >    started popping up from the new helpers so no one else has to
> > > > > > >    troubleshoot that.
> > > > > > >  - Move some accidental DRM_DEBUG_KMS() calls to
> > > > > > > DRM_DEBUG_ATOMIC()
> > > > > > >  - Update documentation again, note that find/release() should
> > > > > > > both
> > > > > > > not
> > > > > > > be
> > > > > > >    called on the same port in a single atomic check phase (but
> > > > > > > multiple
> > > > > > >    calls to one or the other is OK)
> > > > > > > 
> > > > > > > Changes since v4:
> > > > > > >  - Don't skip the atomic checks for VCPI allocations if no new
> > > > > > > VCPI
> > > > > > >    allocations happen in a state. This makes the next change I'm
> > > > > > > about
> > > > > > >    to list here a lot easier to implement.
> > > > > > >  - Don't ignore VCPI allocations on destroyed ports, instead
> > > > > > > ensure
> > > > > > > that
> > > > > > >    when ports are destroyed and still have VCPI allocations in
> > > > > > > the
> > > > > > >    topology state, the only state changes allowed are releasing
> > > > > > > said
> > > > > > >    ports' VCPI. This prevents a state with a mix of VCPI
> > > > > > > allocations
> > > > > > >    from destroyed ports, and allocations from valid ports.
> > > > > > > 
> > > > > > > Changes since v3:
> > > > > > >  - Don't release VCPI allocations in the topology state
> > > > > > > immediately
> > > > > > > in
> > > > > > >    drm_dp_atomic_release_vcpi_slots(), instead mark them as 0
> > > > > > > and
> > > > > > > skip
> > > > > > >    over them in drm_dp_mst_duplicate_state(). This makes it so
> > > > > > >    drm_dp_atomic_release_vcpi_slots() is still idempotent while
> > > > > > > also
> > > > > > >    throwing warnings if the driver messes up it's book keeping
> > > > > > > and
> > > > > > > tries
> > > > > > >    to release VCPI slots on a port that doesn't have any pre-
> > > > > > > existing
> > > > > > >    VCPI allocation - danvet
> > > > > > >  - Change mst_state/state in some debugging messages to "mst
> > > > > > > state"
> > > > > > > 
> > > > > > > Changes since v2:
> > > > > > >  - Use kmemdup() for duplicating MST state - danvet
> > > > > > >  - Move port validation out of duplicate state callback - danvet
> > > > > > >  - Handle looping through MST topology states in
> > > > > > >    drm_dp_mst_atomic_check() so the driver doesn't have to do it
> > > > > > >  - Fix documentation in drm_dp_atomic_find_vcpi_slots()
> > > > > > >  - Move the atomic check for each individual topology state into
> > > > > > > it's
> > > > > > >    own function, reduces indenting
> > > > > > >  - Don't consider "stale" MST ports when calculating the
> > > > > > > bandwidth
> > > > > > >    requirements. This is needed because originally we relied on
> > > > > > > the
> > > > > > >    state duplication functions to prune any stale ports from the
> > > > > > > new
> > > > > > >    state, which would prevent us from incorrectly considering
> > > > > > > their
> > > > > > >    bandwidth requirements alongside legitimate new payloads.
> > > > > > >  - Add function references in drm_dp_atomic_release_vcpi_slots()
> > > > > > > -
> > > > > > > danvet
> > > > > > >  - Annotate atomic VCPI and atomic check functions with
> > > > > > > __must_check
> > > > > > >    - danvet
> > > > > > > 
> > > > > > > Changes since v1:
> > > > > > >  - Don't use the now-removed ->atomic_check() for private
> > > > > > > objects
> > > > > > > hook,
> > > > > > >    just give drivers a function to call themselves
> > > > > > > 
> > > > > > > Signed-off-by: Lyude Paul <[e-mail]>
> > > > > > > Cc: Daniel Vetter <[e-mail]>
> > > > > > > ---
> > > > > > >  drivers/gpu/drm/drm_dp_mst_topology.c | 294
> > > > > > > +++++++++++++++++++++++---
> > > > > > >  drivers/gpu/drm/i915/intel_display.c  |   4 +
> > > > > > >  drivers/gpu/drm/i915/intel_dp_mst.c   |  64 +++---
> > > > > > >  include/drm/drm_dp_mst_helper.h       |  24 ++-
> > > > > > >  4 files changed, 319 insertions(+), 67 deletions(-)
> > > > > > > 
> > > > > > > diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c
> > > > > > > b/drivers/gpu/drm/drm_dp_mst_topology.c
> > > > > > > index 00fbe7a2699d..183d0e832ced 100644
> > > > > > > --- a/drivers/gpu/drm/drm_dp_mst_topology.c
> > > > > > > +++ b/drivers/gpu/drm/drm_dp_mst_topology.c
> > > > > > > @@ -2617,21 +2617,42 @@ static int drm_dp_init_vcpi(struct
> > > > > > > drm_dp_mst_topology_mgr *mgr,
> > > > > > >  }
> > > > > > > 
> > > > > > >  /**
> > > > > > > - * drm_dp_atomic_find_vcpi_slots() - Find and add vcpi slots to
> > > > > > > the
> > > > > > > state
> > > > > > > + * drm_dp_atomic_find_vcpi_slots() - Find and add VCPI slots to
> > > > > > > the
> > > > > > > state
> > > > > > >   * @state: global atomic state
> > > > > > >   * @mgr: MST topology manager for the port
> > > > > > >   * @port: port to find vcpi slots for
> > > > > > >   * @pbn: bandwidth required for the mode in PBN
> > > > > > >   *
> > > > > > > - * RETURNS:
> > > > > > > - * Total slots in the atomic state assigned for this port or
> > > > > > > error
> > > > > > > + * Allocates VCPI slots to @port, replacing any previous VCPI
> > > > > > > allocations it
> > > > > > > + * may have had. Any atomic drivers which support MST must call
> > > > > > > this
> > > > > > > function
> > > > > > > + * in their &drm_encoder_helper_funcs.atomic_check() callback
> > > > > > > to
> > > > > > > change
> > > > > > > the
> > > > > > > + * current VCPI allocation for the new state, but only when
> > > > > > > + * &drm_crtc_state.mode_changed or
> > > > > > > &drm_crtc_state.connectors_changed
> > > > > > > is set
> > > > > > > + * to ensure compatibility with userspace applications that
> > > > > > > still
> > > > > > > use
> > > > > > > the
> > > > > > > + * legacy modesetting UAPI.
> > > > > > > + *
> > > > > > > + * Allocations set by this function are not checked against the
> > > > > > > bandwidth
> > > > > > > + * restraints of @mgr until the driver calls
> > > > > > > drm_dp_mst_atomic_check().
> > > > > > > + *
> > > > > > > + * Additionally, it is OK to call this function multiple times
> > > > > > > on
> > > > > > > the
> > > > > > > same
> > > > > > > + * @port as needed. It is not OK however, to call this function
> > > > > > > and
> > > > > > > + * drm_dp_atomic_release_vcpi_slots() in the same atomic check
> > > > > > > phase.
> > > > > > > + *
> > > > > > > + * See also:
> > > > > > > + * drm_dp_atomic_release_vcpi_slots()
> > > > > > > + * drm_dp_mst_atomic_check()
> > > > > > > + *
> > > > > > > + * Returns:
> > > > > > > + * Total slots in the atomic state assigned for this port, or a
> > > > > > > negative error
> > > > > > > + * code if the port no longer exists
> > > > > > >   */
> > > > > > >  int drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state
> > > > > > > *state,
> > > > > > >  				  struct drm_dp_mst_topology_mgr *mgr,
> > > > > > >  				  struct drm_dp_mst_port *port, int
> > > > > > > pbn)
> > > > > > >  {
> > > > > > >  	struct drm_dp_mst_topology_state *topology_state;
> > > > > > > -	int req_slots;
> > > > > > > +	struct drm_dp_vcpi_allocation *pos, *vcpi = NULL;
> > > > > > > +	int prev_slots, req_slots, ret;
> > > > > > > 
> > > > > > >  	topology_state = drm_atomic_get_mst_topology_state(state,
> > > > > > > mgr);
> > > > > > >  	if (IS_ERR(topology_state))
> > > > > > > @@ -2640,20 +2661,60 @@ int drm_dp_atomic_find_vcpi_slots(struct
> > > > > > > drm_atomic_state *state,
> > > > > > >  	port = drm_dp_get_validated_port_ref(mgr, port);
> > > > > > >  	if (port == NULL)
> > > > > > >  		return -EINVAL;
> > > > > > > -	req_slots = DIV_ROUND_UP(pbn, mgr->pbn_div);
> > > > > > > -	DRM_DEBUG_KMS("vcpi slots req=%d, avail=%d\n",
> > > > > > > -			req_slots, topology_state->avail_slots);
> > > > > > > 
> > > > > > > -	if (req_slots > topology_state->avail_slots) {
> > > > > > > -		drm_dp_put_port(port);
> > > > > > > -		return -ENOSPC;
> > > > > > > +	topology_state->vcpi_allocated = true;
> > > > > > > +
> > > > > > > +	/* Find the current allocation for this port, if any */
> > > > > > > +	list_for_each_entry(pos, &topology_state->vcpis, next) {
> > > > > > > +		if (pos->port == port) {
> > > > > > > +			vcpi = pos;
> > > > > > > +			prev_slots = vcpi->vcpi;
> > > > > > > +
> > > > > > > +			/*
> > > > > > > +			 * This should never happen, unless the driver
> > > > > > > tries
> > > > > > > +			 * releasing and allocating the same VCPI
> > > > > > > allocation,
> > > > > > > +			 * which is an error
> > > > > > > +			 */
> > > > > > > +			if (WARN_ON(!prev_slots)) {
> > > > > > > +				DRM_ERROR("cannot allocate and release
> > > > > > > VCPI on
> > > > > > > [MST PORT:%p] in the same state\n",
> > > > > > > +					  port);
> > > > > > > +				return -EINVAL;
> > > > > > > +			}
> > > > > > 
> > > > > > Allowing atomic changes to output state (like disabling it on one
> > > > > > crtc,
> > > > > > and then enabling on the other, thus both releasing and re-
> > > > > > allocating
> > > > > > the
> > > > > > vcpi in one go) is kinda the point of atomic. So definitely can't
> > > > > > EINVAL
> > > > > > here.
> > > > > > 
> > > > > > Sounds like we need more testcases in igt?
> > > > > > 
> > > > > > > +
> > > > > > > +			break;
> > > > > > > +		}
> > > > > > >  	}
> > > > > > > +	if (!vcpi)
> > > > > > > +		prev_slots = 0;
> > > > > > > 
> > > > > > > -	topology_state->avail_slots -= req_slots;
> > > > > > > -	DRM_DEBUG_KMS("vcpi slots avail=%d", topology_state-
> > > > > > > > avail_slots);
> > > > > > > +	req_slots = DIV_ROUND_UP(pbn, mgr->pbn_div);
> > > > > > > +
> > > > > > > +	DRM_DEBUG_ATOMIC("[CONNECTOR:%d:%s] [MST PORT:%p] VCPI %d ->
> > > > > > > %d\n",
> > > > > > > +			 port->connector->base.id, port->connector-
> > > > > > > > name,
> > > > > > > +			 port, prev_slots, req_slots);
> > > > > > > +
> > > > > > > +	/* Add the new allocation to the state */
> > > > > > > +	if (!vcpi) {
> > > > > > > +		vcpi = kzalloc(sizeof(*vcpi), GFP_KERNEL);
> > > > > > > +		if (!vcpi) {
> > > > > > > +			ret = -ENOMEM;
> > > > > > > +			goto out;
> > > > > > > +		}
> > > > > > > 
> > > > > > > +		vcpi->port = drm_dp_get_validated_port_ref(mgr, port);
> > > > > > > +		if (!vcpi->port) {
> > > > > > > +			ret = -EINVAL;
> > > > > > > +			kfree(vcpi);
> > > > > > > +			goto out;
> > > > > > > +		}
> > > > > > > +		list_add(&vcpi->next, &topology_state->vcpis);
> > > > > > > +	}
> > > > > > > +	vcpi->vcpi = req_slots;
> > > > > > > +
> > > > > > > +	ret = req_slots;
> > > > > > > +out:
> > > > > > >  	drm_dp_put_port(port);
> > > > > > > -	return req_slots;
> > > > > > > +	return ret;
> > > > > > >  }
> > > > > > >  EXPORT_SYMBOL(drm_dp_atomic_find_vcpi_slots);
> > > > > > > 
> > > > > > > @@ -2661,31 +2722,66 @@
> > > > > > > EXPORT_SYMBOL(drm_dp_atomic_find_vcpi_slots);
> > > > > > >   * drm_dp_atomic_release_vcpi_slots() - Release allocated vcpi
> > > > > > > slots
> > > > > > >   * @state: global atomic state
> > > > > > >   * @mgr: MST topology manager for the port
> > > > > > > - * @slots: number of vcpi slots to release
> > > > > > > + * @port: The port to release the VCPI slots from
> > > > > > >   *
> > > > > > > - * RETURNS:
> > > > > > > - * 0 if @slots were added back to &drm_dp_mst_topology_state-
> > > > > > > > avail_slots or
> > > > > > > - * negative error code
> > > > > > > + * Releases any VCPI slots that have been allocated to a port
> > > > > > > in
> > > > > > > the
> > > > > > > atomic
> > > > > > > + * state. Any atomic drivers which support MST must call this
> > > > > > > function
> > > > > > > in
> > > > > > > + * their &drm_connector_helper_funcs.atomic_check() callback
> > > > > > > when
> > > > > > > the
> > > > > > > + * connector will no longer have VCPI allocated (e.g. because
> > > > > > > it's
> > > > > > > CRTC
> > > > > > > was
> > > > > > > + * removed) when it had VCPI allocated in the previous atomic
> > > > > > > state.
> > > > > > > + *
> > > > > > > + * It is OK to call this even if @port has been removed from
> > > > > > > the
> > > > > > > system.
> > > > > > > + * Additionally, it is OK to call this function multiple times
> > > > > > > on
> > > > > > > the
> > > > > > > same
> > > > > > > + * @port as needed. It is not OK however, to call this function
> > > > > > > and
> > > > > > > + * drm_dp_atomic_find_vcpi_slots() on the same @port in a
> > > > > > > single
> > > > > > > atomic
> > > > > > > check
> > > > > > > + * phase.
> > > > > > > + *
> > > > > > > + * See also:
> > > > > > > + * drm_dp_atomic_find_vcpi_slots()
> > > > > > > + * drm_dp_mst_atomic_check()
> > > > > > > + *
> > > > > > > + * Returns:
> > > > > > > + * 0 if all slots for this port were added back to
> > > > > > > + * &drm_dp_mst_topology_state.avail_slots or negative error
> > > > > > > code
> > > > > > >   */
> > > > > > >  int drm_dp_atomic_release_vcpi_slots(struct drm_atomic_state
> > > > > > > *state,
> > > > > > >  				     struct drm_dp_mst_topology_mgr
> > > > > > > *mgr,
> > > > > > > -				     int slots)
> > > > > > > +				     struct drm_dp_mst_port *port)
> > > > > > >  {
> > > > > > >  	struct drm_dp_mst_topology_state *topology_state;
> > > > > > > +	struct drm_dp_vcpi_allocation *pos;
> > > > > > > +	bool found = false;
> > > > > > > 
> > > > > > >  	topology_state = drm_atomic_get_mst_topology_state(state,
> > > > > > > mgr);
> > > > > > >  	if (IS_ERR(topology_state))
> > > > > > >  		return PTR_ERR(topology_state);
> > > > > > > 
> > > > > > > -	/* We cannot rely on port->vcpi.num_slots to update
> > > > > > > -	 * topology_state->avail_slots as the port may not exist if
> > > > > > > the parent
> > > > > > > -	 * branch device was unplugged. This should be fixed by
> > > > > > > tracking
> > > > > > > -	 * per-port slot allocation in drm_dp_mst_topology_state
> > > > > > > instead of
> > > > > > > -	 * depending on the caller to tell us how many slots to
> > > > > > > release.
> > > > > > > -	 */
> > > > > > > -	topology_state->avail_slots += slots;
> > > > > > > -	DRM_DEBUG_KMS("vcpi slots released=%d, avail=%d\n",
> > > > > > > -			slots, topology_state->avail_slots);
> > > > > > > +	list_for_each_entry(pos, &topology_state->vcpis, next) {
> > > > > > > +		if (pos->port == port) {
> > > > > > > +			found = true;
> > > > > > > +			break;
> > > > > > > +		}
> > > > > > > +	}
> > > > > > > +	if (WARN_ON(!found)) {
> > > > > > > +		DRM_ERROR("no VCPI for [MST PORT:%p] found in mst
> > > > > > > state %p\n",
> > > > > > > +			  port, &topology_state->base);
> > > > > > > +		return -EINVAL;
> > > > > > > +	}
> > > > > > > +
> > > > > > > +	DRM_DEBUG_ATOMIC("[MST PORT:%p] VCPI %d -> 0\n", port, pos-
> > > > > > > > vcpi);
> > > > > > > +	if (pos->vcpi) {
> > > > > > > +		/*
> > > > > > > +		 * This should be safe-as the previous state will
> > > > > > > still hold a
> > > > > > > +		 * reference to this port until we swap states. If
> > > > > > > it's not,
> > > > > > > +		 * the driver has a bookkeeping error
> > > > > > > +		 */
> > > > > > > +		if (WARN_ON(drm_dp_put_port(port))) {
> > > > > > 
> > > > > > The usual way to do this (instead of adding a return value to the
> > > > > > normal
> > > > > > _put function) is something like
> > > > > > 
> > > > > > 	kref_put(port, function_that_with_just_a_WARNING_DRM_ERROR)
> > > > > > 
> > > > > > avoids the need for patch 2.
> > > > > > 
> > > > > > > +			DRM_ERROR("cannot allocate and release VCPI on
> > > > > > > [MST
> > > > > > > PORT:%p] in the same state\n",
> > > > > > > +				  port);
> > > > > > > +			return -EINVAL;
> > > > > > > +		}
> > > > > > > +		pos->vcpi = 0;
> > > > > > > +	}
> > > > > > > 
> > > > > > >  	return 0;
> > > > > > >  }
> > > > > > > @@ -3115,15 +3211,45 @@ static void
> > > > > > > drm_dp_destroy_connector_work(struct
> > > > > > > work_struct *work)
> > > > > > >  static struct drm_private_state *
> > > > > > >  drm_dp_mst_duplicate_state(struct drm_private_obj *obj)
> > > > > > >  {
> > > > > > > -	struct drm_dp_mst_topology_state *state;
> > > > > > > +	struct drm_dp_mst_topology_state *state, *old_state =
> > > > > > > +		to_dp_mst_topology_state(obj->state);
> > > > > > > +	struct drm_dp_vcpi_allocation *pos, *vcpi;
> > > > > > > 
> > > > > > > -	state = kmemdup(obj->state, sizeof(*state), GFP_KERNEL);
> > > > > > > +	state = kmemdup(old_state, sizeof(*state), GFP_KERNEL);
> > > > > > >  	if (!state)
> > > > > > >  		return NULL;
> > > > > > > 
> > > > > > >  	__drm_atomic_helper_private_obj_duplicate_state(obj, &state-
> > > > > > > > base);
> > > > > > > 
> > > > > > > +	INIT_LIST_HEAD(&state->vcpis);
> > > > > > > +	state->vcpi_allocated = false;
> > > > > > > +
> > > > > > > +	list_for_each_entry(pos, &old_state->vcpis, next) {
> > > > > > > +		/* Prune leftover freed VCPI allocations */
> > > > > > > +		if (!pos->vcpi)
> > > > > > > +			continue;
> > > > > > > +
> > > > > > > +		vcpi = kmemdup(pos, sizeof(*vcpi), GFP_KERNEL);
> > > > > > > +		if (!vcpi)
> > > > > > > +			goto fail;
> > > > > > > +
> > > > > > > +		/* Take a plain old kref here and don't validate the
> > > > > > > port, as
> > > > > > > +		 * it's topology might not even exist anymore
> > > > > > > +		 */
> > > > > > > +		kref_get(&vcpi->port->kref);
> > > > > > > +		list_add(&vcpi->next, &state->vcpis);
> > > > > > > +	}
> > > > > > > +
> > > > > > >  	return &state->base;
> > > > > > > +
> > > > > > > +fail:
> > > > > > > +	list_for_each_entry_safe(pos, vcpi, &state->vcpis, next) {
> > > > > > > +		drm_dp_put_port(pos->port);
> > > > > > > +		kfree(pos);
> > > > > > > +	}
> > > > > > > +	kfree(state);
> > > > > > > +
> > > > > > > +	return NULL;
> > > > > > >  }
> > > > > > > 
> > > > > > >  static void drm_dp_mst_destroy_state(struct drm_private_obj
> > > > > > > *obj,
> > > > > > > @@ -3131,14 +3257,116 @@ static void
> > > > > > > drm_dp_mst_destroy_state(struct
> > > > > > > drm_private_obj *obj,
> > > > > > >  {
> > > > > > >  	struct drm_dp_mst_topology_state *mst_state =
> > > > > > >  		to_dp_mst_topology_state(state);
> > > > > > > +	struct drm_dp_vcpi_allocation *pos, *tmp;
> > > > > > > +
> > > > > > > +	list_for_each_entry_safe(pos, tmp, &mst_state->vcpis, next) {
> > > > > > > +		/* We only keep references to ports with non-zero
> > > > > > > VCPIs */
> > > > > > > +		if (pos->vcpi)
> > > > > > > +			drm_dp_put_port(pos->port);
> > > > > > > +		kfree(pos);
> > > > > > > +	}
> > > > > > > 
> > > > > > >  	kfree(mst_state);
> > > > > > >  }
> > > > > > > 
> > > > > > > -static const struct drm_private_state_funcs mst_state_funcs = {
> > > > > > > +static inline int
> > > > > > > +drm_dp_mst_atomic_check_topology_state(struct
> > > > > > > drm_dp_mst_topology_mgr
> > > > > > > *mgr,
> > > > > > > +				       struct
> > > > > > > drm_dp_mst_topology_state
> > > > > > > *mst_state)
> > > > > > > +{
> > > > > > > +	struct drm_dp_vcpi_allocation *vcpi;
> > > > > > > +	struct drm_dp_mst_port *port;
> > > > > > > +	int avail_slots = 63, ret;
> > > > > > > +
> > > > > > > +	/* There's no possible scenario where releasing VCPI or
> > > > > > > keeping it the
> > > > > > > +	 * same would make the state invalid
> > > > > > > +	 */
> > > > > > > +	if (!mst_state->vcpi_allocated) {
> > > > > > 
> > > > > > Do we need this? If not I'm always in favour of removing code if
> > > > > > it's
> > > > > > not
> > > > > > needed :-)
> > > > > > 
> > > > > > > +		DRM_DEBUG_ATOMIC("[MST MGR:%p] state %p allocates no
> > > > > > > VCPI,
> > > > > > > check skipped\n",
> > > > > > > +				 mgr, &mst_state->base);
> > > > > > > +		return 0;
> > > > > > > +	}
> > > > > > > +
> > > > > > > +	list_for_each_entry(vcpi, &mst_state->vcpis, next) {
> > > > > > > +		/* Releasing VCPI is always OK-even if the port is
> > > > > > > gone */
> > > > > > > +		if (!vcpi->vcpi) {
> > > > > > > +			DRM_DEBUG_ATOMIC("[MST PORT:%p] releases all
> > > > > > > VCPI
> > > > > > > slots\n",
> > > > > > > +					 vcpi->port);
> > > > > > > +			continue;
> > > > > > > +		}
> > > > > > > +
> > > > > > > +		port = drm_dp_get_validated_port_ref(mgr, vcpi->port);
> > > > > > > +		if (!port) {
> > > > > > > +			DRM_DEBUG_ATOMIC("[MST PORT:%p] is gone but
> > > > > > > still has
> > > > > > > VCPI, cannot add new VCPI\n",
> > > > > > > +					 vcpi->port);
> > > > > > > +			return -EINVAL;
> > > > > > > +		}
> > > > > > 
> > > > > > For find_vcpi we've already done this. For unplugged vcpi, we
> > > > > > can't do
> > > > > > this here I think - otherwise if you enable 2 mst outputs on the
> > > > > > same
> > > > > > branch, and unplug both, old non-atomic userspace can never ever
> > > > > > again
> > > > > > disable them because it can't disable both. The check (to skip
> > > > > > already
> > > > > > deallocated ports) above isn't enough to handle that.
> > > > > > 
> > > > > > > +
> > > > > > > +		DRM_DEBUG_ATOMIC("[MST PORT:%p] requires %d vcpi
> > > > > > > slots\n",
> > > > > > > +				 vcpi->port, vcpi->vcpi);
> > > > > > > +
> > > > > > > +		avail_slots -= vcpi->vcpi;
> > > > > > > +		if (avail_slots < 0) {
> > > > > > > +			DRM_DEBUG_ATOMIC("[MST PORT:%p] not enough
> > > > > > > VCPI slots
> > > > > > > in mst state %p (avail=%d)\n",
> > > > > > > +					 vcpi->port, mst_state,
> > > > > > > +					 avail_slots + vcpi->vcpi);
> > > > > > > +			ret = -ENOSPC;
> > > > > > > +			goto port_fail;
> > > > > > > +		}
> > > > > > > +
> > > > > > > +		drm_dp_put_port(port);
> > > > > > > +	}
> > > > > > > +	DRM_DEBUG_ATOMIC("[MST MGR:%p] mst state %p VCPI avail=%d
> > > > > > > used=%d\n",
> > > > > > > +			 mgr, mst_state, avail_slots,
> > > > > > > +			 63 - avail_slots);
> > > > > > > +
> > > > > > > +	return 0;
> > > > > > > +port_fail:
> > > > > > > +	drm_dp_put_port(port);
> > > > > > > +	return ret;
> > > > > > > +}
> > > > > > > +
> > > > > > > +/**
> > > > > > > + * drm_dp_mst_atomic_check - Check that the new state of an MST
> > > > > > > topology in an
> > > > > > > + * atomic update is valid
> > > > > > > + * @state: Pointer to the new &struct drm_dp_mst_topology_state
> > > > > > > + *
> > > > > > > + * Checks the given topology state for an atomic update to
> > > > > > > ensure
> > > > > > > that
> > > > > > > it's
> > > > > > > + * valid. This includes checking whether there's enough
> > > > > > > bandwidth
> > > > > > > to
> > > > > > > support
> > > > > > > + * the new VCPI allocations in the atomic update.
> > > > > > > + *
> > > > > > > + * Any atomic drivers supporting DP MST must make sure to call
> > > > > > > this
> > > > > > > after
> > > > > > > + * checking the rest of their state in their
> > > > > > > + * &drm_mode_config_funcs.atomic_check() callback.
> > > > > > > + *
> > > > > > > + * See also:
> > > > > > > + * drm_dp_atomic_find_vcpi_slots()
> > > > > > > + * drm_dp_atomic_release_vcpi_slots()
> > > > > > > + *
> > > > > > > + * Returns:
> > > > > > > + *
> > > > > > > + * 0 if the new state is valid, negative error code otherwise.
> > > > > > > + */
> > > > > > > +int drm_dp_mst_atomic_check(struct drm_atomic_state *state)
> > > > > > > +{
> > > > > > > +	struct drm_dp_mst_topology_mgr *mgr;
> > > > > > > +	struct drm_dp_mst_topology_state *mst_state;
> > > > > > > +	int i, ret = 0;
> > > > > > > +
> > > > > > > +	for_each_new_mst_mgr_in_state(state, mgr, mst_state, i) {
> > > > > > > +		ret = drm_dp_mst_atomic_check_topology_state(mgr,
> > > > > > > mst_state);
> > > > > > > +		if (ret)
> > > > > > > +			break;
> > > > > > > +	}
> > > > > > > +
> > > > > > > +	return ret;
> > > > > > > +}
> > > > > > > +EXPORT_SYMBOL(drm_dp_mst_atomic_check);
> > > > > > > +
> > > > > > > +const struct drm_private_state_funcs
> > > > > > > drm_dp_mst_topology_state_funcs =
> > > > > > > {
> > > > > > >  	.atomic_duplicate_state = drm_dp_mst_duplicate_state,
> > > > > > >  	.atomic_destroy_state = drm_dp_mst_destroy_state,
> > > > > > >  };
> > > > > > > +EXPORT_SYMBOL(drm_dp_mst_topology_state_funcs);
> > > > > > > 
> > > > > > >  /**
> > > > > > >   * drm_atomic_get_mst_topology_state: get MST topology state
> > > > > > > @@ -3216,13 +3444,11 @@ int drm_dp_mst_topology_mgr_init(struct
> > > > > > > drm_dp_mst_topology_mgr *mgr,
> > > > > > >  		return -ENOMEM;
> > > > > > > 
> > > > > > >  	mst_state->mgr = mgr;
> > > > > > > -
> > > > > > > -	/* max. time slots - one slot for MTP header */
> > > > > > > -	mst_state->avail_slots = 63;
> > > > > > > +	INIT_LIST_HEAD(&mst_state->vcpis);
> > > > > > > 
> > > > > > >  	drm_atomic_private_obj_init(&mgr->base,
> > > > > > >  				    &mst_state->base,
> > > > > > > -				    &mst_state_funcs);
> > > > > > > +				    &drm_dp_mst_topology_state_funcs);
> > > > > > > 
> > > > > > >  	return 0;
> > > > > > >  }
> > > > > > > diff --git a/drivers/gpu/drm/i915/intel_display.c
> > > > > > > b/drivers/gpu/drm/i915/intel_display.c
> > > > > > > index 132e978227fb..8e85e04a5948 100644
> > > > > > > --- a/drivers/gpu/drm/i915/intel_display.c
> > > > > > > +++ b/drivers/gpu/drm/i915/intel_display.c
> > > > > > > @@ -12592,6 +12592,10 @@ static int intel_atomic_check(struct
> > > > > > > drm_device
> > > > > > > *dev,
> > > > > > >  				       "[modeset]" : "[fastset]");
> > > > > > >  	}
> > > > > > > 
> > > > > > > +	ret = drm_dp_mst_atomic_check(state);
> > > > > > > +	if (ret)
> > > > > > > +		return ret;
> > > > > > > +
> > > > > > >  	if (any_ms) {
> > > > > > >  		ret = intel_modeset_checks(state);
> > > > > > > 
> > > > > > > diff --git a/drivers/gpu/drm/i915/intel_dp_mst.c
> > > > > > > b/drivers/gpu/drm/i915/intel_dp_mst.c
> > > > > > > index 4de247ddf05f..be2d1ac7d87d 100644
> > > > > > > --- a/drivers/gpu/drm/i915/intel_dp_mst.c
> > > > > > > +++ b/drivers/gpu/drm/i915/intel_dp_mst.c
> > > > > > > @@ -41,8 +41,13 @@ static bool
> > > > > > > intel_dp_mst_compute_config(struct
> > > > > > > intel_encoder *encoder,
> > > > > > >  	struct drm_connector *connector = conn_state->connector;
> > > > > > >  	void *port = to_intel_connector(connector)->port;
> > > > > > >  	struct drm_atomic_state *state = pipe_config->base.state;
> > > > > > > +	struct drm_crtc *crtc = pipe_config->base.crtc;
> > > > > > > +	struct drm_crtc_state *old_crtc_state =
> > > > > > > +		drm_atomic_get_old_crtc_state(state, crtc);
> > > > > > > +	struct drm_crtc_state *new_crtc_state = &pipe_config->base;
> > > > > > >  	int bpp;
> > > > > > > -	int lane_count, slots = 0;
> > > > > > > +	int lane_count, slots =
> > > > > > > +		to_intel_crtc_state(old_crtc_state)->dp_m_n.tu;
> > > > > > >  	const struct drm_display_mode *adjusted_mode = &pipe_config-
> > > > > > > > base.adjusted_mode;
> > > > > > >  	int mst_pbn;
> > > > > > >  	bool constant_n = drm_dp_has_quirk(&intel_dp->desc,
> > > > > > > @@ -77,8 +82,12 @@ static bool
> > > > > > > intel_dp_mst_compute_config(struct
> > > > > > > intel_encoder *encoder,
> > > > > > >  	mst_pbn = drm_dp_calc_pbn_mode(adjusted_mode->crtc_clock,
> > > > > > > bpp);
> > > > > > >  	pipe_config->pbn = mst_pbn;
> > > > > > > 
> > > > > > > -	/* Zombie connectors can't have VCPI slots */
> > > > > > > -	if (!drm_connector_is_unregistered(connector)) {
> > > > > > > +	/* Only change VCPI allocation on actual mode changes, to
> > > > > > > prevent us
> > > > > > > +	 * from trying to allocate VCPI to ports that no longer exist
> > > > > > > when we
> > > > > > > +	 * may just be trying to disable DPMS on them
> > > > > > > +	 */
> > > > > > > +	if (new_crtc_state->mode_changed ||
> > > > > > > +	    new_crtc_state->connectors_changed) {
> > > > > > 
> > > > > > Why exactly is this needed? We only call compute_config on the
> > > > > > enocoder
> > > > > > when enabling an output. Not when disabling one. Similar for the
> > > > > > atomic
> > > > > > helpers and the various encoder callbacks (you have the same
> > > > > > change in
> > > > > > the
> > > > > > nouveau code). Did this actually blow up while testing?
> > > > > > 
> > > > > > >  		slots = drm_dp_atomic_find_vcpi_slots(state,
> > > > > > >  						      &intel_dp-
> > > > > > > > mst_mgr,
> > > > > > >  						      port,
> > > > > > > @@ -107,36 +116,39 @@ static bool
> > > > > > > intel_dp_mst_compute_config(struct
> > > > > > > intel_encoder *encoder,
> > > > > > >  	return true;
> > > > > > >  }
> > > > > > > 
> > > > > > > -static int intel_dp_mst_atomic_check(struct drm_connector
> > > > > > > *connector,
> > > > > > > -		struct drm_connector_state *new_conn_state)
> > > > > > > +static int
> > > > > > > +intel_dp_mst_atomic_check(struct drm_connector *connector,
> > > > > > > +			  struct drm_connector_state *new_conn_state)
> > > > > > >  {
> > > > > > > +	struct intel_connector *intel_connector =
> > > > > > > +		to_intel_connector(connector);
> > > > > > > +	struct drm_dp_mst_topology_mgr *mgr =
> > > > > > > +		&intel_connector->mst_port->mst_mgr;
> > > > > > > +	struct drm_dp_mst_port *port = intel_connector->port;
> > > > > > >  	struct drm_atomic_state *state = new_conn_state->state;
> > > > > > > -	struct drm_connector_state *old_conn_state;
> > > > > > > -	struct drm_crtc *old_crtc;
> > > > > > > -	struct drm_crtc_state *crtc_state;
> > > > > > > -	int slots, ret = 0;
> > > > > > > +	struct drm_connector_state *old_conn_state =
> > > > > > > +		drm_atomic_get_old_connector_state(state, connector);
> > > > > > > +	struct drm_crtc *old_crtc = old_conn_state->crtc,
> > > > > > > +			*new_crtc = new_conn_state->crtc;
> > > > > > > +	struct drm_crtc_state *old_crtc_state, *new_crtc_state;
> > > > > > > 
> > > > > > > -	old_conn_state = drm_atomic_get_old_connector_state(state,
> > > > > > > connector);
> > > > > > > -	old_crtc = old_conn_state->crtc;
> > > > > > 
> > > > > > Bikeshed: Reworking all the above hides the real changes quite a
> > > > > > bit.
> > > > > > 
> > > > > > >  	if (!old_crtc)
> > > > > > > -		return ret;
> > > > > > > +		return 0;
> > > > > > > 
> > > > > > > -	crtc_state = drm_atomic_get_new_crtc_state(state, old_crtc);
> > > > > > > -	slots = to_intel_crtc_state(crtc_state)->dp_m_n.tu;
> > > > > > > -	if (drm_atomic_crtc_needs_modeset(crtc_state) && slots > 0) {
> > > > > > > -		struct drm_dp_mst_topology_mgr *mgr;
> > > > > > > -		struct drm_encoder *old_encoder;
> > > > > > > +	old_crtc_state = drm_atomic_get_old_crtc_state(state,
> > > > > > > old_crtc);
> > > > > > > +	if (!old_crtc_state ||
> > > > > > 
> > > > > > !old_crtc_state would be a framework bug. Just remove that check.
> > > > > > 
> > > > > > > +	    !to_intel_crtc_state(old_crtc_state)->dp_m_n.tu)
> > > > > > > +		return 0;
> > > > > > 
> > > > > > Hm, why do we need this? Having a DP port previously enabled with
> > > > > > a 0
> > > > > > tu
> > > > > > sounds like a pretty big driver bug.
> > > > > > > -		old_encoder = old_conn_state->best_encoder;
> > > > > > > -		mgr = &enc_to_mst(old_encoder)->primary->dp.mst_mgr;
> > > > > > > +	/* If we switched CRTCs, clear the previous one's allocation
> > > > > > > */
> > > > > > > +	new_crtc_state = drm_atomic_get_new_crtc_state(state,
> > > > > > > old_crtc);
> > > > > > > +	if (new_crtc_state->connectors_changed && !new_crtc_state-
> > > > > > > > enable)
> > > > > > > +		to_intel_crtc_state(new_crtc_state)->dp_m_n.tu = 0;
> > > > > > 
> > > > > > No idea why we need to clear this here? Shouldn't we just clear
> > > > > > this
> > > > > > unconditionally if we release the allocation.
> > > > > > 
> > > > > > Also naming convention, I'd call this old_crtc_new_state, and
> > > > > > maybe
> > > > > > old_crtc_old_state. To make it clear these are the old/new states
> > > > > > for
> > > > > > old_crtc. Instead of old/new state for the new crtc.
> > > > > > 
> > > > > > > -		ret = drm_dp_atomic_release_vcpi_slots(state, mgr,
> > > > > > > slots);
> > > > > > > -		if (ret)
> > > > > > > -			DRM_DEBUG_KMS("failed releasing %d vcpi
> > > > > > > slots:%d\n",
> > > > > > > slots, ret);
> > > > > > > -		else
> > > > > > > -			to_intel_crtc_state(crtc_state)->dp_m_n.tu =
> > > > > > > 0;
> > > > > > > -	}
> > > > > > > -	return ret;
> > > > > > > +	if (new_crtc)
> > > > > > > +		return 0;
> > > > > > 
> > > > > > Not sure why we shouldn't release this? If we handle this in
> > > > > > helpers,
> > > > > > it
> > > > > > would remove piles of special casing code here I think. Plus
> > > > > > remove
> > > > > > special code in the hlpers.
> > > > > > 
> > > > > > 
> > > > > > > +
> > > > > > > +	return drm_dp_atomic_release_vcpi_slots(state, mgr, port);
> > > > > > >  }
> > > > > > > 
> > > > > > >  static void intel_mst_disable_dp(struct intel_encoder *encoder,
> > > > > > > diff --git a/include/drm/drm_dp_mst_helper.h
> > > > > > > b/include/drm/drm_dp_mst_helper.h
> > > > > > > index 3faceb66f5cb..2392e12b6bce 100644
> > > > > > > --- a/include/drm/drm_dp_mst_helper.h
> > > > > > > +++ b/include/drm/drm_dp_mst_helper.h
> > > > > > > @@ -406,9 +406,16 @@ struct drm_dp_payload {
> > > > > > > 
> > > > > > >  #define to_dp_mst_topology_state(x) container_of(x, struct
> > > > > > > drm_dp_mst_topology_state, base)
> > > > > > > 
> > > > > > > +struct drm_dp_vcpi_allocation {
> > > > > > 
> > > > > > I think a comment here that this pointer holds a full reference
> > > > > > (even
> > > > > > past
> > > > > > when the connector is unplugged) would be good. Otherwise the loop
> > > > > > in
> > > > > > release_vcpi would be buggy.
> > > > > > 
> > > > > > > +	struct drm_dp_mst_port *port;
> > > > > > > +	int vcpi;
> > > > > > > +	struct list_head next;
> > > > > > > +};
> > > > > > > +
> > > > > > >  struct drm_dp_mst_topology_state {
> > > > > > >  	struct drm_private_state base;
> > > > > > > -	int avail_slots;
> > > > > > > +	struct list_head vcpis;
> > > > > > > +	u8 vcpi_allocated;
> > > > > > >  	struct drm_dp_mst_topology_mgr *mgr;
> > > > > > >  };
> > > > > > > 
> > > > > > > @@ -619,14 +626,17 @@ void
> > > > > > > drm_dp_mst_topology_mgr_suspend(struct
> > > > > > > drm_dp_mst_topology_mgr *mgr);
> > > > > > >  int drm_dp_mst_topology_mgr_resume(struct
> > > > > > > drm_dp_mst_topology_mgr
> > > > > > > *mgr);
> > > > > > >  struct drm_dp_mst_topology_state
> > > > > > > *drm_atomic_get_mst_topology_state(struct drm_atomic_state
> > > > > > > *state,
> > > > > > >  								    st
> > > > > > > ruct
> > > > > > > drm_dp_mst_topology_mgr *mgr);
> > > > > > > -int drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state
> > > > > > > *state,
> > > > > > > -				  struct drm_dp_mst_topology_mgr *mgr,
> > > > > > > -				  struct drm_dp_mst_port *port, int
> > > > > > > pbn);
> > > > > > > -int drm_dp_atomic_release_vcpi_slots(struct drm_atomic_state
> > > > > > > *state,
> > > > > > > -				     struct drm_dp_mst_topology_mgr
> > > > > > > *mgr,
> > > > > > > -				     int slots);
> > > > > > > +int __must_check
> > > > > > > +drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state *state,
> > > > > > > +			      struct drm_dp_mst_topology_mgr *mgr,
> > > > > > > +			      struct drm_dp_mst_port *port, int pbn);
> > > > > > > +int __must_check
> > > > > > > +drm_dp_atomic_release_vcpi_slots(struct drm_atomic_state
> > > > > > > *state,
> > > > > > > +				 struct drm_dp_mst_topology_mgr *mgr,
> > > > > > > +				 struct drm_dp_mst_port *port);
> > > > > > >  int drm_dp_send_power_updown_phy(struct drm_dp_mst_topology_mgr
> > > > > > > *mgr,
> > > > > > >  				 struct drm_dp_mst_port *port, bool
> > > > > > > power_up);
> > > > > > > +int __must_check drm_dp_mst_atomic_check(struct
> > > > > > > drm_atomic_state
> > > > > > > *state);
> > > > > > > 
> > > > > > >  extern const struct drm_private_state_funcs
> > > > > > > drm_dp_mst_topology_state_funcs;
> > > > > > > 
> > > > > > > --
> > > > > > > 2.19.1
> > > > > > > 
> > > > > > 
> > > > > > --
> > > > > > Daniel Vetter
> > > > > > Software Engineer, Intel Corporation
> > > > > > http://blog.ffwll.ch
> > > > --
> > > > Cheers,
> > > > 	Lyude Paul
> > > > 
> > --
> > Cheers,
> > 	Lyude Paul
> >

Lyude Paul Nov. 29, 2018, 1:14 a.m. UTC | #10

On Wed, 2018-11-28 at 09:17 +0100, Daniel Vetter wrote:
> On Tue, Nov 27, 2018 at 08:44:14PM -0500, Lyude Paul wrote:
> > On Tue, 2018-11-27 at 20:44 +0100, Daniel Vetter wrote:
> > 
> > We could do this the other way around so it looks like this maybe
> > 
> > struct kref; /* manages kfree() */
> > struct topology_kref; /* corresonds to lifetime in topology */
> > 
> > Then only expose functions for the normal kref to drivers, so that any
> > possible confusion is still limited to drm_dp_mst_topology.c
> 
> I like this bikeshed a lot. Since we need a bunch of the plain
> kref_get/put internally, probably still good to have a wrapper. For that
> the port_malloc_get/put() still sounds good to me - port_kfree_get/put
> sounds confusing, since it's not the kfree we're refcounting, but the
> memory allocation.
> 
> Another option would be to add _topology to the public get/put functions,
> but that makes for a fairly long function name :-/

bleh. Looked at not using a malloc() prefix in the naming but it's
definitely still more confusing without one.

I really do question if we really want the canonical naming prefix for the MST
helpers to be drm_dp_mst_topology. It would be very nice to have this freed so
we could do something like this:

drm_dp_mst_topology_get_mstb() /* &topology_kref */
drm_dp_mst_topology_put_mstb() /* &topology_kref */
drm_dp_mst_get_mstb_malloc()   /* &kref */
drm_dp_mst_put_mstb_malloc()   /* &kref */

drm_dp_mst_topology_get_port() /* &topology_kref */
drm_dp_mst_topology_put_port() /* &topology_kref */
drm_dp_mst_get_port_malloc()   /* &kref */
drm_dp_mst_put_port_malloc()   /* &kref */

Additionally, I had an epiphany and figured out a seriously dead-simple
rule that answers "what ref to use where" in the MST helper code:

Let's start with a topology like this:

  |- mst_primary
     |- port #1
     |- port #2
     |- port #3
        |- mstb #1
	   |- port #4
	      |- mstb #2
	   |- port #5
	   |- port #6
	      |- mstb #3
	         |- port #7
		    |- mstb #4
	         |- port #8
	         |- port #9

If mstb #1 was removed from the topology, ports 4-9 and mstbs 2-4 must
lose their topology references. Simply put: each port holds a topology
reference to it's mstb (if there is one), and each mstb holds a topology
reference to it's ports.

Now, let's allocate some payloads on the topology:

  |- mst_primary
     |- port #1
     |- port #2
     |- port #3
        |- mstb #1
	   |- port #4
	      |- mstb #2 (has payload)
	   |- port #5
	   |- port #6
	      |- mstb #3
	         |- port #7
		    |- mstb #4 (has payload)
	         |- port #8
	         |- port #9

So, now if we remove mstb #1 any payloads under it need to be freed.
Since mstb #4's and #2's parents no longer exist in the topology
however, we must go back up the tree to find our last living relative.

So:

mstb #4 → port #7 → mstb #3 → port #6 → mstb #1 → port #3   for payload 1
mstb #4 → port #4 → mstb #1 → port #3                       for payload 2

Going off this we can come up with the rule for malloc refs: each port
holds a malloc ref to it's parent mstb, and each mstb holds a malloc ref
to it's parent port (if there is one). So, it's just the reverse of the
topology ref rule. Now just add refs for payloads and other stuff, and
we're good.

Hooray! Now I can also use this in any docs I write too :)

> -Daniel
> 
> > > > > -Daniel
> > > > > 
> > > > > > Piles of comments below.
> > > > > > 
> > > > > > Cheers, Daniel
> > > > > > 
> > > > > > >  - Use the small changes to drm_dp_put_port() to add even more
> > > > > > > error
> > > > > > >    checking to make misusage of the helpers more obvious. I
> > > > > > > added
> > > > > > > this
> > > > > > >    after having to chase down various use-after-free conditions
> > > > > > > that
> > > > > > >    started popping up from the new helpers so no one else has to
> > > > > > >    troubleshoot that.
> > > > > > >  - Move some accidental DRM_DEBUG_KMS() calls to
> > > > > > > DRM_DEBUG_ATOMIC()
> > > > > > >  - Update documentation again, note that find/release() should
> > > > > > > both
> > > > > > > not
> > > > > > > be
> > > > > > >    called on the same port in a single atomic check phase (but
> > > > > > > multiple
> > > > > > >    calls to one or the other is OK)
> > > > > > > 
> > > > > > > Changes since v4:
> > > > > > >  - Don't skip the atomic checks for VCPI allocations if no new
> > > > > > > VCPI
> > > > > > >    allocations happen in a state. This makes the next change I'm
> > > > > > > about
> > > > > > >    to list here a lot easier to implement.
> > > > > > >  - Don't ignore VCPI allocations on destroyed ports, instead
> > > > > > > ensure
> > > > > > > that
> > > > > > >    when ports are destroyed and still have VCPI allocations in
> > > > > > > the
> > > > > > >    topology state, the only state changes allowed are releasing
> > > > > > > said
> > > > > > >    ports' VCPI. This prevents a state with a mix of VCPI
> > > > > > > allocations
> > > > > > >    from destroyed ports, and allocations from valid ports.
> > > > > > > 
> > > > > > > Changes since v3:
> > > > > > >  - Don't release VCPI allocations in the topology state
> > > > > > > immediately
> > > > > > > in
> > > > > > >    drm_dp_atomic_release_vcpi_slots(), instead mark them as 0
> > > > > > > and
> > > > > > > skip
> > > > > > >    over them in drm_dp_mst_duplicate_state(). This makes it so
> > > > > > >    drm_dp_atomic_release_vcpi_slots() is still idempotent while
> > > > > > > also
> > > > > > >    throwing warnings if the driver messes up it's book keeping
> > > > > > > and
> > > > > > > tries
> > > > > > >    to release VCPI slots on a port that doesn't have any pre-
> > > > > > > existing
> > > > > > >    VCPI allocation - danvet
> > > > > > >  - Change mst_state/state in some debugging messages to "mst
> > > > > > > state"
> > > > > > > 
> > > > > > > Changes since v2:
> > > > > > >  - Use kmemdup() for duplicating MST state - danvet
> > > > > > >  - Move port validation out of duplicate state callback - danvet
> > > > > > >  - Handle looping through MST topology states in
> > > > > > >    drm_dp_mst_atomic_check() so the driver doesn't have to do it
> > > > > > >  - Fix documentation in drm_dp_atomic_find_vcpi_slots()
> > > > > > >  - Move the atomic check for each individual topology state into
> > > > > > > it's
> > > > > > >    own function, reduces indenting
> > > > > > >  - Don't consider "stale" MST ports when calculating the
> > > > > > > bandwidth
> > > > > > >    requirements. This is needed because originally we relied on
> > > > > > > the
> > > > > > >    state duplication functions to prune any stale ports from the
> > > > > > > new
> > > > > > >    state, which would prevent us from incorrectly considering
> > > > > > > their
> > > > > > >    bandwidth requirements alongside legitimate new payloads.
> > > > > > >  - Add function references in drm_dp_atomic_release_vcpi_slots()
> > > > > > > -
> > > > > > > danvet
> > > > > > >  - Annotate atomic VCPI and atomic check functions with
> > > > > > > __must_check
> > > > > > >    - danvet
> > > > > > > 
> > > > > > > Changes since v1:
> > > > > > >  - Don't use the now-removed ->atomic_check() for private
> > > > > > > objects
> > > > > > > hook,
> > > > > > >    just give drivers a function to call themselves
> > > > > > > 
> > > > > > > Signed-off-by: Lyude Paul <[e-mail]>
> > > > > > > Cc: Daniel Vetter <[e-mail]>
> > > > > > > ---
> > > > > > >  drivers/gpu/drm/drm_dp_mst_topology.c | 294
> > > > > > > +++++++++++++++++++++++---
> > > > > > >  drivers/gpu/drm/i915/intel_display.c  |   4 +
> > > > > > >  drivers/gpu/drm/i915/intel_dp_mst.c   |  64 +++---
> > > > > > >  include/drm/drm_dp_mst_helper.h       |  24 ++-
> > > > > > >  4 files changed, 319 insertions(+), 67 deletions(-)
> > > > > > > 
> > > > > > > diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c
> > > > > > > b/drivers/gpu/drm/drm_dp_mst_topology.c
> > > > > > > index 00fbe7a2699d..183d0e832ced 100644
> > > > > > > --- a/drivers/gpu/drm/drm_dp_mst_topology.c
> > > > > > > +++ b/drivers/gpu/drm/drm_dp_mst_topology.c
> > > > > > > @@ -2617,21 +2617,42 @@ static int drm_dp_init_vcpi(struct
> > > > > > > drm_dp_mst_topology_mgr *mgr,
> > > > > > >  }
> > > > > > > 
> > > > > > >  /**
> > > > > > > - * drm_dp_atomic_find_vcpi_slots() - Find and add vcpi slots to
> > > > > > > the
> > > > > > > state
> > > > > > > + * drm_dp_atomic_find_vcpi_slots() - Find and add VCPI slots to
> > > > > > > the
> > > > > > > state
> > > > > > >   * @state: global atomic state
> > > > > > >   * @mgr: MST topology manager for the port
> > > > > > >   * @port: port to find vcpi slots for
> > > > > > >   * @pbn: bandwidth required for the mode in PBN
> > > > > > >   *
> > > > > > > - * RETURNS:
> > > > > > > - * Total slots in the atomic state assigned for this port or
> > > > > > > error
> > > > > > > + * Allocates VCPI slots to @port, replacing any previous VCPI
> > > > > > > allocations it
> > > > > > > + * may have had. Any atomic drivers which support MST must call
> > > > > > > this
> > > > > > > function
> > > > > > > + * in their &drm_encoder_helper_funcs.atomic_check() callback
> > > > > > > to
> > > > > > > change
> > > > > > > the
> > > > > > > + * current VCPI allocation for the new state, but only when
> > > > > > > + * &drm_crtc_state.mode_changed or
> > > > > > > &drm_crtc_state.connectors_changed
> > > > > > > is set
> > > > > > > + * to ensure compatibility with userspace applications that
> > > > > > > still
> > > > > > > use
> > > > > > > the
> > > > > > > + * legacy modesetting UAPI.
> > > > > > > + *
> > > > > > > + * Allocations set by this function are not checked against the
> > > > > > > bandwidth
> > > > > > > + * restraints of @mgr until the driver calls
> > > > > > > drm_dp_mst_atomic_check().
> > > > > > > + *
> > > > > > > + * Additionally, it is OK to call this function multiple times
> > > > > > > on
> > > > > > > the
> > > > > > > same
> > > > > > > + * @port as needed. It is not OK however, to call this function
> > > > > > > and
> > > > > > > + * drm_dp_atomic_release_vcpi_slots() in the same atomic check
> > > > > > > phase.
> > > > > > > + *
> > > > > > > + * See also:
> > > > > > > + * drm_dp_atomic_release_vcpi_slots()
> > > > > > > + * drm_dp_mst_atomic_check()
> > > > > > > + *
> > > > > > > + * Returns:
> > > > > > > + * Total slots in the atomic state assigned for this port, or a
> > > > > > > negative error
> > > > > > > + * code if the port no longer exists
> > > > > > >   */
> > > > > > >  int drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state
> > > > > > > *state,
> > > > > > >  				  struct drm_dp_mst_topology_mgr *mgr,
> > > > > > >  				  struct drm_dp_mst_port *port, int
> > > > > > > pbn)
> > > > > > >  {
> > > > > > >  	struct drm_dp_mst_topology_state *topology_state;
> > > > > > > -	int req_slots;
> > > > > > > +	struct drm_dp_vcpi_allocation *pos, *vcpi = NULL;
> > > > > > > +	int prev_slots, req_slots, ret;
> > > > > > > 
> > > > > > >  	topology_state = drm_atomic_get_mst_topology_state(state,
> > > > > > > mgr);
> > > > > > >  	if (IS_ERR(topology_state))
> > > > > > > @@ -2640,20 +2661,60 @@ int drm_dp_atomic_find_vcpi_slots(struct
> > > > > > > drm_atomic_state *state,
> > > > > > >  	port = drm_dp_get_validated_port_ref(mgr, port);
> > > > > > >  	if (port == NULL)
> > > > > > >  		return -EINVAL;
> > > > > > > -	req_slots = DIV_ROUND_UP(pbn, mgr->pbn_div);
> > > > > > > -	DRM_DEBUG_KMS("vcpi slots req=%d, avail=%d\n",
> > > > > > > -			req_slots, topology_state->avail_slots);
> > > > > > > 
> > > > > > > -	if (req_slots > topology_state->avail_slots) {
> > > > > > > -		drm_dp_put_port(port);
> > > > > > > -		return -ENOSPC;
> > > > > > > +	topology_state->vcpi_allocated = true;
> > > > > > > +
> > > > > > > +	/* Find the current allocation for this port, if any */
> > > > > > > +	list_for_each_entry(pos, &topology_state->vcpis, next) {
> > > > > > > +		if (pos->port == port) {
> > > > > > > +			vcpi = pos;
> > > > > > > +			prev_slots = vcpi->vcpi;
> > > > > > > +
> > > > > > > +			/*
> > > > > > > +			 * This should never happen, unless the driver
> > > > > > > tries
> > > > > > > +			 * releasing and allocating the same VCPI
> > > > > > > allocation,
> > > > > > > +			 * which is an error
> > > > > > > +			 */
> > > > > > > +			if (WARN_ON(!prev_slots)) {
> > > > > > > +				DRM_ERROR("cannot allocate and release
> > > > > > > VCPI on
> > > > > > > [MST PORT:%p] in the same state\n",
> > > > > > > +					  port);
> > > > > > > +				return -EINVAL;
> > > > > > > +			}
> > > > > > 
> > > > > > Allowing atomic changes to output state (like disabling it on one
> > > > > > crtc,
> > > > > > and then enabling on the other, thus both releasing and re-
> > > > > > allocating
> > > > > > the
> > > > > > vcpi in one go) is kinda the point of atomic. So definitely can't
> > > > > > EINVAL
> > > > > > here.
> > > > > > 
> > > > > > Sounds like we need more testcases in igt?
> > > > > > 
> > > > > > > +
> > > > > > > +			break;
> > > > > > > +		}
> > > > > > >  	}
> > > > > > > +	if (!vcpi)
> > > > > > > +		prev_slots = 0;
> > > > > > > 
> > > > > > > -	topology_state->avail_slots -= req_slots;
> > > > > > > -	DRM_DEBUG_KMS("vcpi slots avail=%d", topology_state-
> > > > > > > > avail_slots);
> > > > > > > +	req_slots = DIV_ROUND_UP(pbn, mgr->pbn_div);
> > > > > > > +
> > > > > > > +	DRM_DEBUG_ATOMIC("[CONNECTOR:%d:%s] [MST PORT:%p] VCPI %d ->
> > > > > > > %d\n",
> > > > > > > +			 port->connector->base.id, port->connector-
> > > > > > > > name,
> > > > > > > +			 port, prev_slots, req_slots);
> > > > > > > +
> > > > > > > +	/* Add the new allocation to the state */
> > > > > > > +	if (!vcpi) {
> > > > > > > +		vcpi = kzalloc(sizeof(*vcpi), GFP_KERNEL);
> > > > > > > +		if (!vcpi) {
> > > > > > > +			ret = -ENOMEM;
> > > > > > > +			goto out;
> > > > > > > +		}
> > > > > > > 
> > > > > > > +		vcpi->port = drm_dp_get_validated_port_ref(mgr, port);
> > > > > > > +		if (!vcpi->port) {
> > > > > > > +			ret = -EINVAL;
> > > > > > > +			kfree(vcpi);
> > > > > > > +			goto out;
> > > > > > > +		}
> > > > > > > +		list_add(&vcpi->next, &topology_state->vcpis);
> > > > > > > +	}
> > > > > > > +	vcpi->vcpi = req_slots;
> > > > > > > +
> > > > > > > +	ret = req_slots;
> > > > > > > +out:
> > > > > > >  	drm_dp_put_port(port);
> > > > > > > -	return req_slots;
> > > > > > > +	return ret;
> > > > > > >  }
> > > > > > >  EXPORT_SYMBOL(drm_dp_atomic_find_vcpi_slots);
> > > > > > > 
> > > > > > > @@ -2661,31 +2722,66 @@
> > > > > > > EXPORT_SYMBOL(drm_dp_atomic_find_vcpi_slots);
> > > > > > >   * drm_dp_atomic_release_vcpi_slots() - Release allocated vcpi
> > > > > > > slots
> > > > > > >   * @state: global atomic state
> > > > > > >   * @mgr: MST topology manager for the port
> > > > > > > - * @slots: number of vcpi slots to release
> > > > > > > + * @port: The port to release the VCPI slots from
> > > > > > >   *
> > > > > > > - * RETURNS:
> > > > > > > - * 0 if @slots were added back to &drm_dp_mst_topology_state-
> > > > > > > > avail_slots or
> > > > > > > - * negative error code
> > > > > > > + * Releases any VCPI slots that have been allocated to a port
> > > > > > > in
> > > > > > > the
> > > > > > > atomic
> > > > > > > + * state. Any atomic drivers which support MST must call this
> > > > > > > function
> > > > > > > in
> > > > > > > + * their &drm_connector_helper_funcs.atomic_check() callback
> > > > > > > when
> > > > > > > the
> > > > > > > + * connector will no longer have VCPI allocated (e.g. because
> > > > > > > it's
> > > > > > > CRTC
> > > > > > > was
> > > > > > > + * removed) when it had VCPI allocated in the previous atomic
> > > > > > > state.
> > > > > > > + *
> > > > > > > + * It is OK to call this even if @port has been removed from
> > > > > > > the
> > > > > > > system.
> > > > > > > + * Additionally, it is OK to call this function multiple times
> > > > > > > on
> > > > > > > the
> > > > > > > same
> > > > > > > + * @port as needed. It is not OK however, to call this function
> > > > > > > and
> > > > > > > + * drm_dp_atomic_find_vcpi_slots() on the same @port in a
> > > > > > > single
> > > > > > > atomic
> > > > > > > check
> > > > > > > + * phase.
> > > > > > > + *
> > > > > > > + * See also:
> > > > > > > + * drm_dp_atomic_find_vcpi_slots()
> > > > > > > + * drm_dp_mst_atomic_check()
> > > > > > > + *
> > > > > > > + * Returns:
> > > > > > > + * 0 if all slots for this port were added back to
> > > > > > > + * &drm_dp_mst_topology_state.avail_slots or negative error
> > > > > > > code
> > > > > > >   */
> > > > > > >  int drm_dp_atomic_release_vcpi_slots(struct drm_atomic_state
> > > > > > > *state,
> > > > > > >  				     struct drm_dp_mst_topology_mgr
> > > > > > > *mgr,
> > > > > > > -				     int slots)
> > > > > > > +				     struct drm_dp_mst_port *port)
> > > > > > >  {
> > > > > > >  	struct drm_dp_mst_topology_state *topology_state;
> > > > > > > +	struct drm_dp_vcpi_allocation *pos;
> > > > > > > +	bool found = false;
> > > > > > > 
> > > > > > >  	topology_state = drm_atomic_get_mst_topology_state(state,
> > > > > > > mgr);
> > > > > > >  	if (IS_ERR(topology_state))
> > > > > > >  		return PTR_ERR(topology_state);
> > > > > > > 
> > > > > > > -	/* We cannot rely on port->vcpi.num_slots to update
> > > > > > > -	 * topology_state->avail_slots as the port may not exist if
> > > > > > > the parent
> > > > > > > -	 * branch device was unplugged. This should be fixed by
> > > > > > > tracking
> > > > > > > -	 * per-port slot allocation in drm_dp_mst_topology_state
> > > > > > > instead of
> > > > > > > -	 * depending on the caller to tell us how many slots to
> > > > > > > release.
> > > > > > > -	 */
> > > > > > > -	topology_state->avail_slots += slots;
> > > > > > > -	DRM_DEBUG_KMS("vcpi slots released=%d, avail=%d\n",
> > > > > > > -			slots, topology_state->avail_slots);
> > > > > > > +	list_for_each_entry(pos, &topology_state->vcpis, next) {
> > > > > > > +		if (pos->port == port) {
> > > > > > > +			found = true;
> > > > > > > +			break;
> > > > > > > +		}
> > > > > > > +	}
> > > > > > > +	if (WARN_ON(!found)) {
> > > > > > > +		DRM_ERROR("no VCPI for [MST PORT:%p] found in mst
> > > > > > > state %p\n",
> > > > > > > +			  port, &topology_state->base);
> > > > > > > +		return -EINVAL;
> > > > > > > +	}
> > > > > > > +
> > > > > > > +	DRM_DEBUG_ATOMIC("[MST PORT:%p] VCPI %d -> 0\n", port, pos-
> > > > > > > > vcpi);
> > > > > > > +	if (pos->vcpi) {
> > > > > > > +		/*
> > > > > > > +		 * This should be safe-as the previous state will
> > > > > > > still hold a
> > > > > > > +		 * reference to this port until we swap states. If
> > > > > > > it's not,
> > > > > > > +		 * the driver has a bookkeeping error
> > > > > > > +		 */
> > > > > > > +		if (WARN_ON(drm_dp_put_port(port))) {
> > > > > > 
> > > > > > The usual way to do this (instead of adding a return value to the
> > > > > > normal
> > > > > > _put function) is something like
> > > > > > 
> > > > > > 	kref_put(port, function_that_with_just_a_WARNING_DRM_ERROR)
> > > > > > 
> > > > > > avoids the need for patch 2.
> > > > > > 
> > > > > > > +			DRM_ERROR("cannot allocate and release VCPI on
> > > > > > > [MST
> > > > > > > PORT:%p] in the same state\n",
> > > > > > > +				  port);
> > > > > > > +			return -EINVAL;
> > > > > > > +		}
> > > > > > > +		pos->vcpi = 0;
> > > > > > > +	}
> > > > > > > 
> > > > > > >  	return 0;
> > > > > > >  }
> > > > > > > @@ -3115,15 +3211,45 @@ static void
> > > > > > > drm_dp_destroy_connector_work(struct
> > > > > > > work_struct *work)
> > > > > > >  static struct drm_private_state *
> > > > > > >  drm_dp_mst_duplicate_state(struct drm_private_obj *obj)
> > > > > > >  {
> > > > > > > -	struct drm_dp_mst_topology_state *state;
> > > > > > > +	struct drm_dp_mst_topology_state *state, *old_state =
> > > > > > > +		to_dp_mst_topology_state(obj->state);
> > > > > > > +	struct drm_dp_vcpi_allocation *pos, *vcpi;
> > > > > > > 
> > > > > > > -	state = kmemdup(obj->state, sizeof(*state), GFP_KERNEL);
> > > > > > > +	state = kmemdup(old_state, sizeof(*state), GFP_KERNEL);
> > > > > > >  	if (!state)
> > > > > > >  		return NULL;
> > > > > > > 
> > > > > > >  	__drm_atomic_helper_private_obj_duplicate_state(obj, &state-
> > > > > > > > base);
> > > > > > > 
> > > > > > > +	INIT_LIST_HEAD(&state->vcpis);
> > > > > > > +	state->vcpi_allocated = false;
> > > > > > > +
> > > > > > > +	list_for_each_entry(pos, &old_state->vcpis, next) {
> > > > > > > +		/* Prune leftover freed VCPI allocations */
> > > > > > > +		if (!pos->vcpi)
> > > > > > > +			continue;
> > > > > > > +
> > > > > > > +		vcpi = kmemdup(pos, sizeof(*vcpi), GFP_KERNEL);
> > > > > > > +		if (!vcpi)
> > > > > > > +			goto fail;
> > > > > > > +
> > > > > > > +		/* Take a plain old kref here and don't validate the
> > > > > > > port, as
> > > > > > > +		 * it's topology might not even exist anymore
> > > > > > > +		 */
> > > > > > > +		kref_get(&vcpi->port->kref);
> > > > > > > +		list_add(&vcpi->next, &state->vcpis);
> > > > > > > +	}
> > > > > > > +
> > > > > > >  	return &state->base;
> > > > > > > +
> > > > > > > +fail:
> > > > > > > +	list_for_each_entry_safe(pos, vcpi, &state->vcpis, next) {
> > > > > > > +		drm_dp_put_port(pos->port);
> > > > > > > +		kfree(pos);
> > > > > > > +	}
> > > > > > > +	kfree(state);
> > > > > > > +
> > > > > > > +	return NULL;
> > > > > > >  }
> > > > > > > 
> > > > > > >  static void drm_dp_mst_destroy_state(struct drm_private_obj
> > > > > > > *obj,
> > > > > > > @@ -3131,14 +3257,116 @@ static void
> > > > > > > drm_dp_mst_destroy_state(struct
> > > > > > > drm_private_obj *obj,
> > > > > > >  {
> > > > > > >  	struct drm_dp_mst_topology_state *mst_state =
> > > > > > >  		to_dp_mst_topology_state(state);
> > > > > > > +	struct drm_dp_vcpi_allocation *pos, *tmp;
> > > > > > > +
> > > > > > > +	list_for_each_entry_safe(pos, tmp, &mst_state->vcpis, next) {
> > > > > > > +		/* We only keep references to ports with non-zero
> > > > > > > VCPIs */
> > > > > > > +		if (pos->vcpi)
> > > > > > > +			drm_dp_put_port(pos->port);
> > > > > > > +		kfree(pos);
> > > > > > > +	}
> > > > > > > 
> > > > > > >  	kfree(mst_state);
> > > > > > >  }
> > > > > > > 
> > > > > > > -static const struct drm_private_state_funcs mst_state_funcs = {
> > > > > > > +static inline int
> > > > > > > +drm_dp_mst_atomic_check_topology_state(struct
> > > > > > > drm_dp_mst_topology_mgr
> > > > > > > *mgr,
> > > > > > > +				       struct
> > > > > > > drm_dp_mst_topology_state
> > > > > > > *mst_state)
> > > > > > > +{
> > > > > > > +	struct drm_dp_vcpi_allocation *vcpi;
> > > > > > > +	struct drm_dp_mst_port *port;
> > > > > > > +	int avail_slots = 63, ret;
> > > > > > > +
> > > > > > > +	/* There's no possible scenario where releasing VCPI or
> > > > > > > keeping it the
> > > > > > > +	 * same would make the state invalid
> > > > > > > +	 */
> > > > > > > +	if (!mst_state->vcpi_allocated) {
> > > > > > 
> > > > > > Do we need this? If not I'm always in favour of removing code if
> > > > > > it's
> > > > > > not
> > > > > > needed :-)
> > > > > > 
> > > > > > > +		DRM_DEBUG_ATOMIC("[MST MGR:%p] state %p allocates no
> > > > > > > VCPI,
> > > > > > > check skipped\n",
> > > > > > > +				 mgr, &mst_state->base);
> > > > > > > +		return 0;
> > > > > > > +	}
> > > > > > > +
> > > > > > > +	list_for_each_entry(vcpi, &mst_state->vcpis, next) {
> > > > > > > +		/* Releasing VCPI is always OK-even if the port is
> > > > > > > gone */
> > > > > > > +		if (!vcpi->vcpi) {
> > > > > > > +			DRM_DEBUG_ATOMIC("[MST PORT:%p] releases all
> > > > > > > VCPI
> > > > > > > slots\n",
> > > > > > > +					 vcpi->port);
> > > > > > > +			continue;
> > > > > > > +		}
> > > > > > > +
> > > > > > > +		port = drm_dp_get_validated_port_ref(mgr, vcpi->port);
> > > > > > > +		if (!port) {
> > > > > > > +			DRM_DEBUG_ATOMIC("[MST PORT:%p] is gone but
> > > > > > > still has
> > > > > > > VCPI, cannot add new VCPI\n",
> > > > > > > +					 vcpi->port);
> > > > > > > +			return -EINVAL;
> > > > > > > +		}
> > > > > > 
> > > > > > For find_vcpi we've already done this. For unplugged vcpi, we
> > > > > > can't do
> > > > > > this here I think - otherwise if you enable 2 mst outputs on the
> > > > > > same
> > > > > > branch, and unplug both, old non-atomic userspace can never ever
> > > > > > again
> > > > > > disable them because it can't disable both. The check (to skip
> > > > > > already
> > > > > > deallocated ports) above isn't enough to handle that.
> > > > > > 
> > > > > > > +
> > > > > > > +		DRM_DEBUG_ATOMIC("[MST PORT:%p] requires %d vcpi
> > > > > > > slots\n",
> > > > > > > +				 vcpi->port, vcpi->vcpi);
> > > > > > > +
> > > > > > > +		avail_slots -= vcpi->vcpi;
> > > > > > > +		if (avail_slots < 0) {
> > > > > > > +			DRM_DEBUG_ATOMIC("[MST PORT:%p] not enough
> > > > > > > VCPI slots
> > > > > > > in mst state %p (avail=%d)\n",
> > > > > > > +					 vcpi->port, mst_state,
> > > > > > > +					 avail_slots + vcpi->vcpi);
> > > > > > > +			ret = -ENOSPC;
> > > > > > > +			goto port_fail;
> > > > > > > +		}
> > > > > > > +
> > > > > > > +		drm_dp_put_port(port);
> > > > > > > +	}
> > > > > > > +	DRM_DEBUG_ATOMIC("[MST MGR:%p] mst state %p VCPI avail=%d
> > > > > > > used=%d\n",
> > > > > > > +			 mgr, mst_state, avail_slots,
> > > > > > > +			 63 - avail_slots);
> > > > > > > +
> > > > > > > +	return 0;
> > > > > > > +port_fail:
> > > > > > > +	drm_dp_put_port(port);
> > > > > > > +	return ret;
> > > > > > > +}
> > > > > > > +
> > > > > > > +/**
> > > > > > > + * drm_dp_mst_atomic_check - Check that the new state of an MST
> > > > > > > topology in an
> > > > > > > + * atomic update is valid
> > > > > > > + * @state: Pointer to the new &struct drm_dp_mst_topology_state
> > > > > > > + *
> > > > > > > + * Checks the given topology state for an atomic update to
> > > > > > > ensure
> > > > > > > that
> > > > > > > it's
> > > > > > > + * valid. This includes checking whether there's enough
> > > > > > > bandwidth
> > > > > > > to
> > > > > > > support
> > > > > > > + * the new VCPI allocations in the atomic update.
> > > > > > > + *
> > > > > > > + * Any atomic drivers supporting DP MST must make sure to call
> > > > > > > this
> > > > > > > after
> > > > > > > + * checking the rest of their state in their
> > > > > > > + * &drm_mode_config_funcs.atomic_check() callback.
> > > > > > > + *
> > > > > > > + * See also:
> > > > > > > + * drm_dp_atomic_find_vcpi_slots()
> > > > > > > + * drm_dp_atomic_release_vcpi_slots()
> > > > > > > + *
> > > > > > > + * Returns:
> > > > > > > + *
> > > > > > > + * 0 if the new state is valid, negative error code otherwise.
> > > > > > > + */
> > > > > > > +int drm_dp_mst_atomic_check(struct drm_atomic_state *state)
> > > > > > > +{
> > > > > > > +	struct drm_dp_mst_topology_mgr *mgr;
> > > > > > > +	struct drm_dp_mst_topology_state *mst_state;
> > > > > > > +	int i, ret = 0;
> > > > > > > +
> > > > > > > +	for_each_new_mst_mgr_in_state(state, mgr, mst_state, i) {
> > > > > > > +		ret = drm_dp_mst_atomic_check_topology_state(mgr,
> > > > > > > mst_state);
> > > > > > > +		if (ret)
> > > > > > > +			break;
> > > > > > > +	}
> > > > > > > +
> > > > > > > +	return ret;
> > > > > > > +}
> > > > > > > +EXPORT_SYMBOL(drm_dp_mst_atomic_check);
> > > > > > > +
> > > > > > > +const struct drm_private_state_funcs
> > > > > > > drm_dp_mst_topology_state_funcs =
> > > > > > > {
> > > > > > >  	.atomic_duplicate_state = drm_dp_mst_duplicate_state,
> > > > > > >  	.atomic_destroy_state = drm_dp_mst_destroy_state,
> > > > > > >  };
> > > > > > > +EXPORT_SYMBOL(drm_dp_mst_topology_state_funcs);
> > > > > > > 
> > > > > > >  /**
> > > > > > >   * drm_atomic_get_mst_topology_state: get MST topology state
> > > > > > > @@ -3216,13 +3444,11 @@ int drm_dp_mst_topology_mgr_init(struct
> > > > > > > drm_dp_mst_topology_mgr *mgr,
> > > > > > >  		return -ENOMEM;
> > > > > > > 
> > > > > > >  	mst_state->mgr = mgr;
> > > > > > > -
> > > > > > > -	/* max. time slots - one slot for MTP header */
> > > > > > > -	mst_state->avail_slots = 63;
> > > > > > > +	INIT_LIST_HEAD(&mst_state->vcpis);
> > > > > > > 
> > > > > > >  	drm_atomic_private_obj_init(&mgr->base,
> > > > > > >  				    &mst_state->base,
> > > > > > > -				    &mst_state_funcs);
> > > > > > > +				    &drm_dp_mst_topology_state_funcs);
> > > > > > > 
> > > > > > >  	return 0;
> > > > > > >  }
> > > > > > > diff --git a/drivers/gpu/drm/i915/intel_display.c
> > > > > > > b/drivers/gpu/drm/i915/intel_display.c
> > > > > > > index 132e978227fb..8e85e04a5948 100644
> > > > > > > --- a/drivers/gpu/drm/i915/intel_display.c
> > > > > > > +++ b/drivers/gpu/drm/i915/intel_display.c
> > > > > > > @@ -12592,6 +12592,10 @@ static int intel_atomic_check(struct
> > > > > > > drm_device
> > > > > > > *dev,
> > > > > > >  				       "[modeset]" : "[fastset]");
> > > > > > >  	}
> > > > > > > 
> > > > > > > +	ret = drm_dp_mst_atomic_check(state);
> > > > > > > +	if (ret)
> > > > > > > +		return ret;
> > > > > > > +
> > > > > > >  	if (any_ms) {
> > > > > > >  		ret = intel_modeset_checks(state);
> > > > > > > 
> > > > > > > diff --git a/drivers/gpu/drm/i915/intel_dp_mst.c
> > > > > > > b/drivers/gpu/drm/i915/intel_dp_mst.c
> > > > > > > index 4de247ddf05f..be2d1ac7d87d 100644
> > > > > > > --- a/drivers/gpu/drm/i915/intel_dp_mst.c
> > > > > > > +++ b/drivers/gpu/drm/i915/intel_dp_mst.c
> > > > > > > @@ -41,8 +41,13 @@ static bool
> > > > > > > intel_dp_mst_compute_config(struct
> > > > > > > intel_encoder *encoder,
> > > > > > >  	struct drm_connector *connector = conn_state->connector;
> > > > > > >  	void *port = to_intel_connector(connector)->port;
> > > > > > >  	struct drm_atomic_state *state = pipe_config->base.state;
> > > > > > > +	struct drm_crtc *crtc = pipe_config->base.crtc;
> > > > > > > +	struct drm_crtc_state *old_crtc_state =
> > > > > > > +		drm_atomic_get_old_crtc_state(state, crtc);
> > > > > > > +	struct drm_crtc_state *new_crtc_state = &pipe_config->base;
> > > > > > >  	int bpp;
> > > > > > > -	int lane_count, slots = 0;
> > > > > > > +	int lane_count, slots =
> > > > > > > +		to_intel_crtc_state(old_crtc_state)->dp_m_n.tu;
> > > > > > >  	const struct drm_display_mode *adjusted_mode = &pipe_config-
> > > > > > > > base.adjusted_mode;
> > > > > > >  	int mst_pbn;
> > > > > > >  	bool constant_n = drm_dp_has_quirk(&intel_dp->desc,
> > > > > > > @@ -77,8 +82,12 @@ static bool
> > > > > > > intel_dp_mst_compute_config(struct
> > > > > > > intel_encoder *encoder,
> > > > > > >  	mst_pbn = drm_dp_calc_pbn_mode(adjusted_mode->crtc_clock,
> > > > > > > bpp);
> > > > > > >  	pipe_config->pbn = mst_pbn;
> > > > > > > 
> > > > > > > -	/* Zombie connectors can't have VCPI slots */
> > > > > > > -	if (!drm_connector_is_unregistered(connector)) {
> > > > > > > +	/* Only change VCPI allocation on actual mode changes, to
> > > > > > > prevent us
> > > > > > > +	 * from trying to allocate VCPI to ports that no longer exist
> > > > > > > when we
> > > > > > > +	 * may just be trying to disable DPMS on them
> > > > > > > +	 */
> > > > > > > +	if (new_crtc_state->mode_changed ||
> > > > > > > +	    new_crtc_state->connectors_changed) {
> > > > > > 
> > > > > > Why exactly is this needed? We only call compute_config on the
> > > > > > enocoder
> > > > > > when enabling an output. Not when disabling one. Similar for the
> > > > > > atomic
> > > > > > helpers and the various encoder callbacks (you have the same
> > > > > > change in
> > > > > > the
> > > > > > nouveau code). Did this actually blow up while testing?
> > > > > > 
> > > > > > >  		slots = drm_dp_atomic_find_vcpi_slots(state,
> > > > > > >  						      &intel_dp-
> > > > > > > > mst_mgr,
> > > > > > >  						      port,
> > > > > > > @@ -107,36 +116,39 @@ static bool
> > > > > > > intel_dp_mst_compute_config(struct
> > > > > > > intel_encoder *encoder,
> > > > > > >  	return true;
> > > > > > >  }
> > > > > > > 
> > > > > > > -static int intel_dp_mst_atomic_check(struct drm_connector
> > > > > > > *connector,
> > > > > > > -		struct drm_connector_state *new_conn_state)
> > > > > > > +static int
> > > > > > > +intel_dp_mst_atomic_check(struct drm_connector *connector,
> > > > > > > +			  struct drm_connector_state *new_conn_state)
> > > > > > >  {
> > > > > > > +	struct intel_connector *intel_connector =
> > > > > > > +		to_intel_connector(connector);
> > > > > > > +	struct drm_dp_mst_topology_mgr *mgr =
> > > > > > > +		&intel_connector->mst_port->mst_mgr;
> > > > > > > +	struct drm_dp_mst_port *port = intel_connector->port;
> > > > > > >  	struct drm_atomic_state *state = new_conn_state->state;
> > > > > > > -	struct drm_connector_state *old_conn_state;
> > > > > > > -	struct drm_crtc *old_crtc;
> > > > > > > -	struct drm_crtc_state *crtc_state;
> > > > > > > -	int slots, ret = 0;
> > > > > > > +	struct drm_connector_state *old_conn_state =
> > > > > > > +		drm_atomic_get_old_connector_state(state, connector);
> > > > > > > +	struct drm_crtc *old_crtc = old_conn_state->crtc,
> > > > > > > +			*new_crtc = new_conn_state->crtc;
> > > > > > > +	struct drm_crtc_state *old_crtc_state, *new_crtc_state;
> > > > > > > 
> > > > > > > -	old_conn_state = drm_atomic_get_old_connector_state(state,
> > > > > > > connector);
> > > > > > > -	old_crtc = old_conn_state->crtc;
> > > > > > 
> > > > > > Bikeshed: Reworking all the above hides the real changes quite a
> > > > > > bit.
> > > > > > 
> > > > > > >  	if (!old_crtc)
> > > > > > > -		return ret;
> > > > > > > +		return 0;
> > > > > > > 
> > > > > > > -	crtc_state = drm_atomic_get_new_crtc_state(state, old_crtc);
> > > > > > > -	slots = to_intel_crtc_state(crtc_state)->dp_m_n.tu;
> > > > > > > -	if (drm_atomic_crtc_needs_modeset(crtc_state) && slots > 0) {
> > > > > > > -		struct drm_dp_mst_topology_mgr *mgr;
> > > > > > > -		struct drm_encoder *old_encoder;
> > > > > > > +	old_crtc_state = drm_atomic_get_old_crtc_state(state,
> > > > > > > old_crtc);
> > > > > > > +	if (!old_crtc_state ||
> > > > > > 
> > > > > > !old_crtc_state would be a framework bug. Just remove that check.
> > > > > > 
> > > > > > > +	    !to_intel_crtc_state(old_crtc_state)->dp_m_n.tu)
> > > > > > > +		return 0;
> > > > > > 
> > > > > > Hm, why do we need this? Having a DP port previously enabled with
> > > > > > a 0
> > > > > > tu
> > > > > > sounds like a pretty big driver bug.
> > > > > > > -		old_encoder = old_conn_state->best_encoder;
> > > > > > > -		mgr = &enc_to_mst(old_encoder)->primary->dp.mst_mgr;
> > > > > > > +	/* If we switched CRTCs, clear the previous one's allocation
> > > > > > > */
> > > > > > > +	new_crtc_state = drm_atomic_get_new_crtc_state(state,
> > > > > > > old_crtc);
> > > > > > > +	if (new_crtc_state->connectors_changed && !new_crtc_state-
> > > > > > > > enable)
> > > > > > > +		to_intel_crtc_state(new_crtc_state)->dp_m_n.tu = 0;
> > > > > > 
> > > > > > No idea why we need to clear this here? Shouldn't we just clear
> > > > > > this
> > > > > > unconditionally if we release the allocation.
> > > > > > 
> > > > > > Also naming convention, I'd call this old_crtc_new_state, and
> > > > > > maybe
> > > > > > old_crtc_old_state. To make it clear these are the old/new states
> > > > > > for
> > > > > > old_crtc. Instead of old/new state for the new crtc.
> > > > > > 
> > > > > > > -		ret = drm_dp_atomic_release_vcpi_slots(state, mgr,
> > > > > > > slots);
> > > > > > > -		if (ret)
> > > > > > > -			DRM_DEBUG_KMS("failed releasing %d vcpi
> > > > > > > slots:%d\n",
> > > > > > > slots, ret);
> > > > > > > -		else
> > > > > > > -			to_intel_crtc_state(crtc_state)->dp_m_n.tu =
> > > > > > > 0;
> > > > > > > -	}
> > > > > > > -	return ret;
> > > > > > > +	if (new_crtc)
> > > > > > > +		return 0;
> > > > > > 
> > > > > > Not sure why we shouldn't release this? If we handle this in
> > > > > > helpers,
> > > > > > it
> > > > > > would remove piles of special casing code here I think. Plus
> > > > > > remove
> > > > > > special code in the hlpers.
> > > > > > 
> > > > > > 
> > > > > > > +
> > > > > > > +	return drm_dp_atomic_release_vcpi_slots(state, mgr, port);
> > > > > > >  }
> > > > > > > 
> > > > > > >  static void intel_mst_disable_dp(struct intel_encoder *encoder,
> > > > > > > diff --git a/include/drm/drm_dp_mst_helper.h
> > > > > > > b/include/drm/drm_dp_mst_helper.h
> > > > > > > index 3faceb66f5cb..2392e12b6bce 100644
> > > > > > > --- a/include/drm/drm_dp_mst_helper.h
> > > > > > > +++ b/include/drm/drm_dp_mst_helper.h
> > > > > > > @@ -406,9 +406,16 @@ struct drm_dp_payload {
> > > > > > > 
> > > > > > >  #define to_dp_mst_topology_state(x) container_of(x, struct
> > > > > > > drm_dp_mst_topology_state, base)
> > > > > > > 
> > > > > > > +struct drm_dp_vcpi_allocation {
> > > > > > 
> > > > > > I think a comment here that this pointer holds a full reference
> > > > > > (even
> > > > > > past
> > > > > > when the connector is unplugged) would be good. Otherwise the loop
> > > > > > in
> > > > > > release_vcpi would be buggy.
> > > > > > 
> > > > > > > +	struct drm_dp_mst_port *port;
> > > > > > > +	int vcpi;
> > > > > > > +	struct list_head next;
> > > > > > > +};
> > > > > > > +
> > > > > > >  struct drm_dp_mst_topology_state {
> > > > > > >  	struct drm_private_state base;
> > > > > > > -	int avail_slots;
> > > > > > > +	struct list_head vcpis;
> > > > > > > +	u8 vcpi_allocated;
> > > > > > >  	struct drm_dp_mst_topology_mgr *mgr;
> > > > > > >  };
> > > > > > > 
> > > > > > > @@ -619,14 +626,17 @@ void
> > > > > > > drm_dp_mst_topology_mgr_suspend(struct
> > > > > > > drm_dp_mst_topology_mgr *mgr);
> > > > > > >  int drm_dp_mst_topology_mgr_resume(struct
> > > > > > > drm_dp_mst_topology_mgr
> > > > > > > *mgr);
> > > > > > >  struct drm_dp_mst_topology_state
> > > > > > > *drm_atomic_get_mst_topology_state(struct drm_atomic_state
> > > > > > > *state,
> > > > > > >  								    st
> > > > > > > ruct
> > > > > > > drm_dp_mst_topology_mgr *mgr);
> > > > > > > -int drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state
> > > > > > > *state,
> > > > > > > -				  struct drm_dp_mst_topology_mgr *mgr,
> > > > > > > -				  struct drm_dp_mst_port *port, int
> > > > > > > pbn);
> > > > > > > -int drm_dp_atomic_release_vcpi_slots(struct drm_atomic_state
> > > > > > > *state,
> > > > > > > -				     struct drm_dp_mst_topology_mgr
> > > > > > > *mgr,
> > > > > > > -				     int slots);
> > > > > > > +int __must_check
> > > > > > > +drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state *state,
> > > > > > > +			      struct drm_dp_mst_topology_mgr *mgr,
> > > > > > > +			      struct drm_dp_mst_port *port, int pbn);
> > > > > > > +int __must_check
> > > > > > > +drm_dp_atomic_release_vcpi_slots(struct drm_atomic_state
> > > > > > > *state,
> > > > > > > +				 struct drm_dp_mst_topology_mgr *mgr,
> > > > > > > +				 struct drm_dp_mst_port *port);
> > > > > > >  int drm_dp_send_power_updown_phy(struct drm_dp_mst_topology_mgr
> > > > > > > *mgr,
> > > > > > >  				 struct drm_dp_mst_port *port, bool
> > > > > > > power_up);
> > > > > > > +int __must_check drm_dp_mst_atomic_check(struct
> > > > > > > drm_atomic_state
> > > > > > > *state);
> > > > > > > 
> > > > > > >  extern const struct drm_private_state_funcs
> > > > > > > drm_dp_mst_topology_state_funcs;
> > > > > > > 
> > > > > > > --
> > > > > > > 2.19.1
> > > > > > > 
> > > > > > 
> > > > > > --
> > > > > > Daniel Vetter
> > > > > > Software Engineer, Intel Corporation
> > > > > > http://blog.ffwll.ch
> > > > --
> > > > Cheers,
> > > > 	Lyude Paul
> > > > 
> > --
> > Cheers,
> > 	Lyude Paul
> >

Daniel Vetter Nov. 29, 2018, 9:21 a.m. UTC | #11

On Wed, Nov 28, 2018 at 08:14:13PM -0500, Lyude Paul wrote:
> 
> On Wed, 2018-11-28 at 09:17 +0100, Daniel Vetter wrote:
> > On Tue, Nov 27, 2018 at 08:44:14PM -0500, Lyude Paul wrote:
> > > On Tue, 2018-11-27 at 20:44 +0100, Daniel Vetter wrote:
> > > 
> > > We could do this the other way around so it looks like this maybe
> > > 
> > > struct kref; /* manages kfree() */
> > > struct topology_kref; /* corresonds to lifetime in topology */
> > > 
> > > Then only expose functions for the normal kref to drivers, so that any
> > > possible confusion is still limited to drm_dp_mst_topology.c
> > 
> > I like this bikeshed a lot. Since we need a bunch of the plain
> > kref_get/put internally, probably still good to have a wrapper. For that
> > the port_malloc_get/put() still sounds good to me - port_kfree_get/put
> > sounds confusing, since it's not the kfree we're refcounting, but the
> > memory allocation.
> > 
> > Another option would be to add _topology to the public get/put functions,
> > but that makes for a fairly long function name :-/
> 
> bleh. Looked at not using a malloc() prefix in the naming but it's
> definitely still more confusing without one.
> 
> I really do question if we really want the canonical naming prefix for the MST
> helpers to be drm_dp_mst_topology. It would be very nice to have this freed so
> we could do something like this:
> 
> drm_dp_mst_topology_get_mstb() /* &topology_kref */
> drm_dp_mst_topology_put_mstb() /* &topology_kref */
> drm_dp_mst_get_mstb_malloc()   /* &kref */
> drm_dp_mst_put_mstb_malloc()   /* &kref */
> 
> drm_dp_mst_topology_get_port() /* &topology_kref */
> drm_dp_mst_topology_put_port() /* &topology_kref */
> drm_dp_mst_get_port_malloc()   /* &kref */
> drm_dp_mst_put_port_malloc()   /* &kref */

Ack on this bikeshed, looks good to me.

> Additionally, I had an epiphany and figured out a seriously dead-simple
> rule that answers "what ref to use where" in the MST helper code:
> 
> Let's start with a topology like this:
> 
>   |- mst_primary
>      |- port #1
>      |- port #2
>      |- port #3
>         |- mstb #1
> 	   |- port #4
> 	      |- mstb #2
> 	   |- port #5
> 	   |- port #6
> 	      |- mstb #3
> 	         |- port #7
> 		    |- mstb #4
> 	         |- port #8
> 	         |- port #9
> 
> If mstb #1 was removed from the topology, ports 4-9 and mstbs 2-4 must
> lose their topology references. Simply put: each port holds a topology
> reference to it's mstb (if there is one), and each mstb holds a topology
> reference to it's ports.
> 
> Now, let's allocate some payloads on the topology:
> 
>   |- mst_primary
>      |- port #1
>      |- port #2
>      |- port #3
>         |- mstb #1
> 	   |- port #4
> 	      |- mstb #2 (has payload)
> 	   |- port #5
> 	   |- port #6
> 	      |- mstb #3
> 	         |- port #7
> 		    |- mstb #4 (has payload)
> 	         |- port #8
> 	         |- port #9
> 
> So, now if we remove mstb #1 any payloads under it need to be freed.
> Since mstb #4's and #2's parents no longer exist in the topology
> however, we must go back up the tree to find our last living relative.
> 
> So:
> 
> mstb #4 → port #7 → mstb #3 → port #6 → mstb #1 → port #3   for payload 1
> mstb #4 → port #4 → mstb #1 → port #3                       for payload 2
> 
> Going off this we can come up with the rule for malloc refs: each port
> holds a malloc ref to it's parent mstb, and each mstb holds a malloc ref
> to it's parent port (if there is one). So, it's just the reverse of the
> topology ref rule. Now just add refs for payloads and other stuff, and
> we're good.
> 
> Hooray! Now I can also use this in any docs I write too :)

This flew right over my head (but coffee isn't working yet I guess), so
definitely good to put this into the kerneldoc comments :-)
-Daniel

> 
> > -Daniel
> > 
> > > > > > -Daniel
> > > > > > 
> > > > > > > Piles of comments below.
> > > > > > > 
> > > > > > > Cheers, Daniel
> > > > > > > 
> > > > > > > >  - Use the small changes to drm_dp_put_port() to add even more
> > > > > > > > error
> > > > > > > >    checking to make misusage of the helpers more obvious. I
> > > > > > > > added
> > > > > > > > this
> > > > > > > >    after having to chase down various use-after-free conditions
> > > > > > > > that
> > > > > > > >    started popping up from the new helpers so no one else has to
> > > > > > > >    troubleshoot that.
> > > > > > > >  - Move some accidental DRM_DEBUG_KMS() calls to
> > > > > > > > DRM_DEBUG_ATOMIC()
> > > > > > > >  - Update documentation again, note that find/release() should
> > > > > > > > both
> > > > > > > > not
> > > > > > > > be
> > > > > > > >    called on the same port in a single atomic check phase (but
> > > > > > > > multiple
> > > > > > > >    calls to one or the other is OK)
> > > > > > > > 
> > > > > > > > Changes since v4:
> > > > > > > >  - Don't skip the atomic checks for VCPI allocations if no new
> > > > > > > > VCPI
> > > > > > > >    allocations happen in a state. This makes the next change I'm
> > > > > > > > about
> > > > > > > >    to list here a lot easier to implement.
> > > > > > > >  - Don't ignore VCPI allocations on destroyed ports, instead
> > > > > > > > ensure
> > > > > > > > that
> > > > > > > >    when ports are destroyed and still have VCPI allocations in
> > > > > > > > the
> > > > > > > >    topology state, the only state changes allowed are releasing
> > > > > > > > said
> > > > > > > >    ports' VCPI. This prevents a state with a mix of VCPI
> > > > > > > > allocations
> > > > > > > >    from destroyed ports, and allocations from valid ports.
> > > > > > > > 
> > > > > > > > Changes since v3:
> > > > > > > >  - Don't release VCPI allocations in the topology state
> > > > > > > > immediately
> > > > > > > > in
> > > > > > > >    drm_dp_atomic_release_vcpi_slots(), instead mark them as 0
> > > > > > > > and
> > > > > > > > skip
> > > > > > > >    over them in drm_dp_mst_duplicate_state(). This makes it so
> > > > > > > >    drm_dp_atomic_release_vcpi_slots() is still idempotent while
> > > > > > > > also
> > > > > > > >    throwing warnings if the driver messes up it's book keeping
> > > > > > > > and
> > > > > > > > tries
> > > > > > > >    to release VCPI slots on a port that doesn't have any pre-
> > > > > > > > existing
> > > > > > > >    VCPI allocation - danvet
> > > > > > > >  - Change mst_state/state in some debugging messages to "mst
> > > > > > > > state"
> > > > > > > > 
> > > > > > > > Changes since v2:
> > > > > > > >  - Use kmemdup() for duplicating MST state - danvet
> > > > > > > >  - Move port validation out of duplicate state callback - danvet
> > > > > > > >  - Handle looping through MST topology states in
> > > > > > > >    drm_dp_mst_atomic_check() so the driver doesn't have to do it
> > > > > > > >  - Fix documentation in drm_dp_atomic_find_vcpi_slots()
> > > > > > > >  - Move the atomic check for each individual topology state into
> > > > > > > > it's
> > > > > > > >    own function, reduces indenting
> > > > > > > >  - Don't consider "stale" MST ports when calculating the
> > > > > > > > bandwidth
> > > > > > > >    requirements. This is needed because originally we relied on
> > > > > > > > the
> > > > > > > >    state duplication functions to prune any stale ports from the
> > > > > > > > new
> > > > > > > >    state, which would prevent us from incorrectly considering
> > > > > > > > their
> > > > > > > >    bandwidth requirements alongside legitimate new payloads.
> > > > > > > >  - Add function references in drm_dp_atomic_release_vcpi_slots()
> > > > > > > > -
> > > > > > > > danvet
> > > > > > > >  - Annotate atomic VCPI and atomic check functions with
> > > > > > > > __must_check
> > > > > > > >    - danvet
> > > > > > > > 
> > > > > > > > Changes since v1:
> > > > > > > >  - Don't use the now-removed ->atomic_check() for private
> > > > > > > > objects
> > > > > > > > hook,
> > > > > > > >    just give drivers a function to call themselves
> > > > > > > > 
> > > > > > > > Signed-off-by: Lyude Paul <[e-mail]>
> > > > > > > > Cc: Daniel Vetter <[e-mail]>
> > > > > > > > ---
> > > > > > > >  drivers/gpu/drm/drm_dp_mst_topology.c | 294
> > > > > > > > +++++++++++++++++++++++---
> > > > > > > >  drivers/gpu/drm/i915/intel_display.c  |   4 +
> > > > > > > >  drivers/gpu/drm/i915/intel_dp_mst.c   |  64 +++---
> > > > > > > >  include/drm/drm_dp_mst_helper.h       |  24 ++-
> > > > > > > >  4 files changed, 319 insertions(+), 67 deletions(-)
> > > > > > > > 
> > > > > > > > diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c
> > > > > > > > b/drivers/gpu/drm/drm_dp_mst_topology.c
> > > > > > > > index 00fbe7a2699d..183d0e832ced 100644
> > > > > > > > --- a/drivers/gpu/drm/drm_dp_mst_topology.c
> > > > > > > > +++ b/drivers/gpu/drm/drm_dp_mst_topology.c
> > > > > > > > @@ -2617,21 +2617,42 @@ static int drm_dp_init_vcpi(struct
> > > > > > > > drm_dp_mst_topology_mgr *mgr,
> > > > > > > >  }
> > > > > > > > 
> > > > > > > >  /**
> > > > > > > > - * drm_dp_atomic_find_vcpi_slots() - Find and add vcpi slots to
> > > > > > > > the
> > > > > > > > state
> > > > > > > > + * drm_dp_atomic_find_vcpi_slots() - Find and add VCPI slots to
> > > > > > > > the
> > > > > > > > state
> > > > > > > >   * @state: global atomic state
> > > > > > > >   * @mgr: MST topology manager for the port
> > > > > > > >   * @port: port to find vcpi slots for
> > > > > > > >   * @pbn: bandwidth required for the mode in PBN
> > > > > > > >   *
> > > > > > > > - * RETURNS:
> > > > > > > > - * Total slots in the atomic state assigned for this port or
> > > > > > > > error
> > > > > > > > + * Allocates VCPI slots to @port, replacing any previous VCPI
> > > > > > > > allocations it
> > > > > > > > + * may have had. Any atomic drivers which support MST must call
> > > > > > > > this
> > > > > > > > function
> > > > > > > > + * in their &drm_encoder_helper_funcs.atomic_check() callback
> > > > > > > > to
> > > > > > > > change
> > > > > > > > the
> > > > > > > > + * current VCPI allocation for the new state, but only when
> > > > > > > > + * &drm_crtc_state.mode_changed or
> > > > > > > > &drm_crtc_state.connectors_changed
> > > > > > > > is set
> > > > > > > > + * to ensure compatibility with userspace applications that
> > > > > > > > still
> > > > > > > > use
> > > > > > > > the
> > > > > > > > + * legacy modesetting UAPI.
> > > > > > > > + *
> > > > > > > > + * Allocations set by this function are not checked against the
> > > > > > > > bandwidth
> > > > > > > > + * restraints of @mgr until the driver calls
> > > > > > > > drm_dp_mst_atomic_check().
> > > > > > > > + *
> > > > > > > > + * Additionally, it is OK to call this function multiple times
> > > > > > > > on
> > > > > > > > the
> > > > > > > > same
> > > > > > > > + * @port as needed. It is not OK however, to call this function
> > > > > > > > and
> > > > > > > > + * drm_dp_atomic_release_vcpi_slots() in the same atomic check
> > > > > > > > phase.
> > > > > > > > + *
> > > > > > > > + * See also:
> > > > > > > > + * drm_dp_atomic_release_vcpi_slots()
> > > > > > > > + * drm_dp_mst_atomic_check()
> > > > > > > > + *
> > > > > > > > + * Returns:
> > > > > > > > + * Total slots in the atomic state assigned for this port, or a
> > > > > > > > negative error
> > > > > > > > + * code if the port no longer exists
> > > > > > > >   */
> > > > > > > >  int drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state
> > > > > > > > *state,
> > > > > > > >  				  struct drm_dp_mst_topology_mgr *mgr,
> > > > > > > >  				  struct drm_dp_mst_port *port, int
> > > > > > > > pbn)
> > > > > > > >  {
> > > > > > > >  	struct drm_dp_mst_topology_state *topology_state;
> > > > > > > > -	int req_slots;
> > > > > > > > +	struct drm_dp_vcpi_allocation *pos, *vcpi = NULL;
> > > > > > > > +	int prev_slots, req_slots, ret;
> > > > > > > > 
> > > > > > > >  	topology_state = drm_atomic_get_mst_topology_state(state,
> > > > > > > > mgr);
> > > > > > > >  	if (IS_ERR(topology_state))
> > > > > > > > @@ -2640,20 +2661,60 @@ int drm_dp_atomic_find_vcpi_slots(struct
> > > > > > > > drm_atomic_state *state,
> > > > > > > >  	port = drm_dp_get_validated_port_ref(mgr, port);
> > > > > > > >  	if (port == NULL)
> > > > > > > >  		return -EINVAL;
> > > > > > > > -	req_slots = DIV_ROUND_UP(pbn, mgr->pbn_div);
> > > > > > > > -	DRM_DEBUG_KMS("vcpi slots req=%d, avail=%d\n",
> > > > > > > > -			req_slots, topology_state->avail_slots);
> > > > > > > > 
> > > > > > > > -	if (req_slots > topology_state->avail_slots) {
> > > > > > > > -		drm_dp_put_port(port);
> > > > > > > > -		return -ENOSPC;
> > > > > > > > +	topology_state->vcpi_allocated = true;
> > > > > > > > +
> > > > > > > > +	/* Find the current allocation for this port, if any */
> > > > > > > > +	list_for_each_entry(pos, &topology_state->vcpis, next) {
> > > > > > > > +		if (pos->port == port) {
> > > > > > > > +			vcpi = pos;
> > > > > > > > +			prev_slots = vcpi->vcpi;
> > > > > > > > +
> > > > > > > > +			/*
> > > > > > > > +			 * This should never happen, unless the driver
> > > > > > > > tries
> > > > > > > > +			 * releasing and allocating the same VCPI
> > > > > > > > allocation,
> > > > > > > > +			 * which is an error
> > > > > > > > +			 */
> > > > > > > > +			if (WARN_ON(!prev_slots)) {
> > > > > > > > +				DRM_ERROR("cannot allocate and release
> > > > > > > > VCPI on
> > > > > > > > [MST PORT:%p] in the same state\n",
> > > > > > > > +					  port);
> > > > > > > > +				return -EINVAL;
> > > > > > > > +			}
> > > > > > > 
> > > > > > > Allowing atomic changes to output state (like disabling it on one
> > > > > > > crtc,
> > > > > > > and then enabling on the other, thus both releasing and re-
> > > > > > > allocating
> > > > > > > the
> > > > > > > vcpi in one go) is kinda the point of atomic. So definitely can't
> > > > > > > EINVAL
> > > > > > > here.
> > > > > > > 
> > > > > > > Sounds like we need more testcases in igt?
> > > > > > > 
> > > > > > > > +
> > > > > > > > +			break;
> > > > > > > > +		}
> > > > > > > >  	}
> > > > > > > > +	if (!vcpi)
> > > > > > > > +		prev_slots = 0;
> > > > > > > > 
> > > > > > > > -	topology_state->avail_slots -= req_slots;
> > > > > > > > -	DRM_DEBUG_KMS("vcpi slots avail=%d", topology_state-
> > > > > > > > > avail_slots);
> > > > > > > > +	req_slots = DIV_ROUND_UP(pbn, mgr->pbn_div);
> > > > > > > > +
> > > > > > > > +	DRM_DEBUG_ATOMIC("[CONNECTOR:%d:%s] [MST PORT:%p] VCPI %d ->
> > > > > > > > %d\n",
> > > > > > > > +			 port->connector->base.id, port->connector-
> > > > > > > > > name,
> > > > > > > > +			 port, prev_slots, req_slots);
> > > > > > > > +
> > > > > > > > +	/* Add the new allocation to the state */
> > > > > > > > +	if (!vcpi) {
> > > > > > > > +		vcpi = kzalloc(sizeof(*vcpi), GFP_KERNEL);
> > > > > > > > +		if (!vcpi) {
> > > > > > > > +			ret = -ENOMEM;
> > > > > > > > +			goto out;
> > > > > > > > +		}
> > > > > > > > 
> > > > > > > > +		vcpi->port = drm_dp_get_validated_port_ref(mgr, port);
> > > > > > > > +		if (!vcpi->port) {
> > > > > > > > +			ret = -EINVAL;
> > > > > > > > +			kfree(vcpi);
> > > > > > > > +			goto out;
> > > > > > > > +		}
> > > > > > > > +		list_add(&vcpi->next, &topology_state->vcpis);
> > > > > > > > +	}
> > > > > > > > +	vcpi->vcpi = req_slots;
> > > > > > > > +
> > > > > > > > +	ret = req_slots;
> > > > > > > > +out:
> > > > > > > >  	drm_dp_put_port(port);
> > > > > > > > -	return req_slots;
> > > > > > > > +	return ret;
> > > > > > > >  }
> > > > > > > >  EXPORT_SYMBOL(drm_dp_atomic_find_vcpi_slots);
> > > > > > > > 
> > > > > > > > @@ -2661,31 +2722,66 @@
> > > > > > > > EXPORT_SYMBOL(drm_dp_atomic_find_vcpi_slots);
> > > > > > > >   * drm_dp_atomic_release_vcpi_slots() - Release allocated vcpi
> > > > > > > > slots
> > > > > > > >   * @state: global atomic state
> > > > > > > >   * @mgr: MST topology manager for the port
> > > > > > > > - * @slots: number of vcpi slots to release
> > > > > > > > + * @port: The port to release the VCPI slots from
> > > > > > > >   *
> > > > > > > > - * RETURNS:
> > > > > > > > - * 0 if @slots were added back to &drm_dp_mst_topology_state-
> > > > > > > > > avail_slots or
> > > > > > > > - * negative error code
> > > > > > > > + * Releases any VCPI slots that have been allocated to a port
> > > > > > > > in
> > > > > > > > the
> > > > > > > > atomic
> > > > > > > > + * state. Any atomic drivers which support MST must call this
> > > > > > > > function
> > > > > > > > in
> > > > > > > > + * their &drm_connector_helper_funcs.atomic_check() callback
> > > > > > > > when
> > > > > > > > the
> > > > > > > > + * connector will no longer have VCPI allocated (e.g. because
> > > > > > > > it's
> > > > > > > > CRTC
> > > > > > > > was
> > > > > > > > + * removed) when it had VCPI allocated in the previous atomic
> > > > > > > > state.
> > > > > > > > + *
> > > > > > > > + * It is OK to call this even if @port has been removed from
> > > > > > > > the
> > > > > > > > system.
> > > > > > > > + * Additionally, it is OK to call this function multiple times
> > > > > > > > on
> > > > > > > > the
> > > > > > > > same
> > > > > > > > + * @port as needed. It is not OK however, to call this function
> > > > > > > > and
> > > > > > > > + * drm_dp_atomic_find_vcpi_slots() on the same @port in a
> > > > > > > > single
> > > > > > > > atomic
> > > > > > > > check
> > > > > > > > + * phase.
> > > > > > > > + *
> > > > > > > > + * See also:
> > > > > > > > + * drm_dp_atomic_find_vcpi_slots()
> > > > > > > > + * drm_dp_mst_atomic_check()
> > > > > > > > + *
> > > > > > > > + * Returns:
> > > > > > > > + * 0 if all slots for this port were added back to
> > > > > > > > + * &drm_dp_mst_topology_state.avail_slots or negative error
> > > > > > > > code
> > > > > > > >   */
> > > > > > > >  int drm_dp_atomic_release_vcpi_slots(struct drm_atomic_state
> > > > > > > > *state,
> > > > > > > >  				     struct drm_dp_mst_topology_mgr
> > > > > > > > *mgr,
> > > > > > > > -				     int slots)
> > > > > > > > +				     struct drm_dp_mst_port *port)
> > > > > > > >  {
> > > > > > > >  	struct drm_dp_mst_topology_state *topology_state;
> > > > > > > > +	struct drm_dp_vcpi_allocation *pos;
> > > > > > > > +	bool found = false;
> > > > > > > > 
> > > > > > > >  	topology_state = drm_atomic_get_mst_topology_state(state,
> > > > > > > > mgr);
> > > > > > > >  	if (IS_ERR(topology_state))
> > > > > > > >  		return PTR_ERR(topology_state);
> > > > > > > > 
> > > > > > > > -	/* We cannot rely on port->vcpi.num_slots to update
> > > > > > > > -	 * topology_state->avail_slots as the port may not exist if
> > > > > > > > the parent
> > > > > > > > -	 * branch device was unplugged. This should be fixed by
> > > > > > > > tracking
> > > > > > > > -	 * per-port slot allocation in drm_dp_mst_topology_state
> > > > > > > > instead of
> > > > > > > > -	 * depending on the caller to tell us how many slots to
> > > > > > > > release.
> > > > > > > > -	 */
> > > > > > > > -	topology_state->avail_slots += slots;
> > > > > > > > -	DRM_DEBUG_KMS("vcpi slots released=%d, avail=%d\n",
> > > > > > > > -			slots, topology_state->avail_slots);
> > > > > > > > +	list_for_each_entry(pos, &topology_state->vcpis, next) {
> > > > > > > > +		if (pos->port == port) {
> > > > > > > > +			found = true;
> > > > > > > > +			break;
> > > > > > > > +		}
> > > > > > > > +	}
> > > > > > > > +	if (WARN_ON(!found)) {
> > > > > > > > +		DRM_ERROR("no VCPI for [MST PORT:%p] found in mst
> > > > > > > > state %p\n",
> > > > > > > > +			  port, &topology_state->base);
> > > > > > > > +		return -EINVAL;
> > > > > > > > +	}
> > > > > > > > +
> > > > > > > > +	DRM_DEBUG_ATOMIC("[MST PORT:%p] VCPI %d -> 0\n", port, pos-
> > > > > > > > > vcpi);
> > > > > > > > +	if (pos->vcpi) {
> > > > > > > > +		/*
> > > > > > > > +		 * This should be safe-as the previous state will
> > > > > > > > still hold a
> > > > > > > > +		 * reference to this port until we swap states. If
> > > > > > > > it's not,
> > > > > > > > +		 * the driver has a bookkeeping error
> > > > > > > > +		 */
> > > > > > > > +		if (WARN_ON(drm_dp_put_port(port))) {
> > > > > > > 
> > > > > > > The usual way to do this (instead of adding a return value to the
> > > > > > > normal
> > > > > > > _put function) is something like
> > > > > > > 
> > > > > > > 	kref_put(port, function_that_with_just_a_WARNING_DRM_ERROR)
> > > > > > > 
> > > > > > > avoids the need for patch 2.
> > > > > > > 
> > > > > > > > +			DRM_ERROR("cannot allocate and release VCPI on
> > > > > > > > [MST
> > > > > > > > PORT:%p] in the same state\n",
> > > > > > > > +				  port);
> > > > > > > > +			return -EINVAL;
> > > > > > > > +		}
> > > > > > > > +		pos->vcpi = 0;
> > > > > > > > +	}
> > > > > > > > 
> > > > > > > >  	return 0;
> > > > > > > >  }
> > > > > > > > @@ -3115,15 +3211,45 @@ static void
> > > > > > > > drm_dp_destroy_connector_work(struct
> > > > > > > > work_struct *work)
> > > > > > > >  static struct drm_private_state *
> > > > > > > >  drm_dp_mst_duplicate_state(struct drm_private_obj *obj)
> > > > > > > >  {
> > > > > > > > -	struct drm_dp_mst_topology_state *state;
> > > > > > > > +	struct drm_dp_mst_topology_state *state, *old_state =
> > > > > > > > +		to_dp_mst_topology_state(obj->state);
> > > > > > > > +	struct drm_dp_vcpi_allocation *pos, *vcpi;
> > > > > > > > 
> > > > > > > > -	state = kmemdup(obj->state, sizeof(*state), GFP_KERNEL);
> > > > > > > > +	state = kmemdup(old_state, sizeof(*state), GFP_KERNEL);
> > > > > > > >  	if (!state)
> > > > > > > >  		return NULL;
> > > > > > > > 
> > > > > > > >  	__drm_atomic_helper_private_obj_duplicate_state(obj, &state-
> > > > > > > > > base);
> > > > > > > > 
> > > > > > > > +	INIT_LIST_HEAD(&state->vcpis);
> > > > > > > > +	state->vcpi_allocated = false;
> > > > > > > > +
> > > > > > > > +	list_for_each_entry(pos, &old_state->vcpis, next) {
> > > > > > > > +		/* Prune leftover freed VCPI allocations */
> > > > > > > > +		if (!pos->vcpi)
> > > > > > > > +			continue;
> > > > > > > > +
> > > > > > > > +		vcpi = kmemdup(pos, sizeof(*vcpi), GFP_KERNEL);
> > > > > > > > +		if (!vcpi)
> > > > > > > > +			goto fail;
> > > > > > > > +
> > > > > > > > +		/* Take a plain old kref here and don't validate the
> > > > > > > > port, as
> > > > > > > > +		 * it's topology might not even exist anymore
> > > > > > > > +		 */
> > > > > > > > +		kref_get(&vcpi->port->kref);
> > > > > > > > +		list_add(&vcpi->next, &state->vcpis);
> > > > > > > > +	}
> > > > > > > > +
> > > > > > > >  	return &state->base;
> > > > > > > > +
> > > > > > > > +fail:
> > > > > > > > +	list_for_each_entry_safe(pos, vcpi, &state->vcpis, next) {
> > > > > > > > +		drm_dp_put_port(pos->port);
> > > > > > > > +		kfree(pos);
> > > > > > > > +	}
> > > > > > > > +	kfree(state);
> > > > > > > > +
> > > > > > > > +	return NULL;
> > > > > > > >  }
> > > > > > > > 
> > > > > > > >  static void drm_dp_mst_destroy_state(struct drm_private_obj
> > > > > > > > *obj,
> > > > > > > > @@ -3131,14 +3257,116 @@ static void
> > > > > > > > drm_dp_mst_destroy_state(struct
> > > > > > > > drm_private_obj *obj,
> > > > > > > >  {
> > > > > > > >  	struct drm_dp_mst_topology_state *mst_state =
> > > > > > > >  		to_dp_mst_topology_state(state);
> > > > > > > > +	struct drm_dp_vcpi_allocation *pos, *tmp;
> > > > > > > > +
> > > > > > > > +	list_for_each_entry_safe(pos, tmp, &mst_state->vcpis, next) {
> > > > > > > > +		/* We only keep references to ports with non-zero
> > > > > > > > VCPIs */
> > > > > > > > +		if (pos->vcpi)
> > > > > > > > +			drm_dp_put_port(pos->port);
> > > > > > > > +		kfree(pos);
> > > > > > > > +	}
> > > > > > > > 
> > > > > > > >  	kfree(mst_state);
> > > > > > > >  }
> > > > > > > > 
> > > > > > > > -static const struct drm_private_state_funcs mst_state_funcs = {
> > > > > > > > +static inline int
> > > > > > > > +drm_dp_mst_atomic_check_topology_state(struct
> > > > > > > > drm_dp_mst_topology_mgr
> > > > > > > > *mgr,
> > > > > > > > +				       struct
> > > > > > > > drm_dp_mst_topology_state
> > > > > > > > *mst_state)
> > > > > > > > +{
> > > > > > > > +	struct drm_dp_vcpi_allocation *vcpi;
> > > > > > > > +	struct drm_dp_mst_port *port;
> > > > > > > > +	int avail_slots = 63, ret;
> > > > > > > > +
> > > > > > > > +	/* There's no possible scenario where releasing VCPI or
> > > > > > > > keeping it the
> > > > > > > > +	 * same would make the state invalid
> > > > > > > > +	 */
> > > > > > > > +	if (!mst_state->vcpi_allocated) {
> > > > > > > 
> > > > > > > Do we need this? If not I'm always in favour of removing code if
> > > > > > > it's
> > > > > > > not
> > > > > > > needed :-)
> > > > > > > 
> > > > > > > > +		DRM_DEBUG_ATOMIC("[MST MGR:%p] state %p allocates no
> > > > > > > > VCPI,
> > > > > > > > check skipped\n",
> > > > > > > > +				 mgr, &mst_state->base);
> > > > > > > > +		return 0;
> > > > > > > > +	}
> > > > > > > > +
> > > > > > > > +	list_for_each_entry(vcpi, &mst_state->vcpis, next) {
> > > > > > > > +		/* Releasing VCPI is always OK-even if the port is
> > > > > > > > gone */
> > > > > > > > +		if (!vcpi->vcpi) {
> > > > > > > > +			DRM_DEBUG_ATOMIC("[MST PORT:%p] releases all
> > > > > > > > VCPI
> > > > > > > > slots\n",
> > > > > > > > +					 vcpi->port);
> > > > > > > > +			continue;
> > > > > > > > +		}
> > > > > > > > +
> > > > > > > > +		port = drm_dp_get_validated_port_ref(mgr, vcpi->port);
> > > > > > > > +		if (!port) {
> > > > > > > > +			DRM_DEBUG_ATOMIC("[MST PORT:%p] is gone but
> > > > > > > > still has
> > > > > > > > VCPI, cannot add new VCPI\n",
> > > > > > > > +					 vcpi->port);
> > > > > > > > +			return -EINVAL;
> > > > > > > > +		}
> > > > > > > 
> > > > > > > For find_vcpi we've already done this. For unplugged vcpi, we
> > > > > > > can't do
> > > > > > > this here I think - otherwise if you enable 2 mst outputs on the
> > > > > > > same
> > > > > > > branch, and unplug both, old non-atomic userspace can never ever
> > > > > > > again
> > > > > > > disable them because it can't disable both. The check (to skip
> > > > > > > already
> > > > > > > deallocated ports) above isn't enough to handle that.
> > > > > > > 
> > > > > > > > +
> > > > > > > > +		DRM_DEBUG_ATOMIC("[MST PORT:%p] requires %d vcpi
> > > > > > > > slots\n",
> > > > > > > > +				 vcpi->port, vcpi->vcpi);
> > > > > > > > +
> > > > > > > > +		avail_slots -= vcpi->vcpi;
> > > > > > > > +		if (avail_slots < 0) {
> > > > > > > > +			DRM_DEBUG_ATOMIC("[MST PORT:%p] not enough
> > > > > > > > VCPI slots
> > > > > > > > in mst state %p (avail=%d)\n",
> > > > > > > > +					 vcpi->port, mst_state,
> > > > > > > > +					 avail_slots + vcpi->vcpi);
> > > > > > > > +			ret = -ENOSPC;
> > > > > > > > +			goto port_fail;
> > > > > > > > +		}
> > > > > > > > +
> > > > > > > > +		drm_dp_put_port(port);
> > > > > > > > +	}
> > > > > > > > +	DRM_DEBUG_ATOMIC("[MST MGR:%p] mst state %p VCPI avail=%d
> > > > > > > > used=%d\n",
> > > > > > > > +			 mgr, mst_state, avail_slots,
> > > > > > > > +			 63 - avail_slots);
> > > > > > > > +
> > > > > > > > +	return 0;
> > > > > > > > +port_fail:
> > > > > > > > +	drm_dp_put_port(port);
> > > > > > > > +	return ret;
> > > > > > > > +}
> > > > > > > > +
> > > > > > > > +/**
> > > > > > > > + * drm_dp_mst_atomic_check - Check that the new state of an MST
> > > > > > > > topology in an
> > > > > > > > + * atomic update is valid
> > > > > > > > + * @state: Pointer to the new &struct drm_dp_mst_topology_state
> > > > > > > > + *
> > > > > > > > + * Checks the given topology state for an atomic update to
> > > > > > > > ensure
> > > > > > > > that
> > > > > > > > it's
> > > > > > > > + * valid. This includes checking whether there's enough
> > > > > > > > bandwidth
> > > > > > > > to
> > > > > > > > support
> > > > > > > > + * the new VCPI allocations in the atomic update.
> > > > > > > > + *
> > > > > > > > + * Any atomic drivers supporting DP MST must make sure to call
> > > > > > > > this
> > > > > > > > after
> > > > > > > > + * checking the rest of their state in their
> > > > > > > > + * &drm_mode_config_funcs.atomic_check() callback.
> > > > > > > > + *
> > > > > > > > + * See also:
> > > > > > > > + * drm_dp_atomic_find_vcpi_slots()
> > > > > > > > + * drm_dp_atomic_release_vcpi_slots()
> > > > > > > > + *
> > > > > > > > + * Returns:
> > > > > > > > + *
> > > > > > > > + * 0 if the new state is valid, negative error code otherwise.
> > > > > > > > + */
> > > > > > > > +int drm_dp_mst_atomic_check(struct drm_atomic_state *state)
> > > > > > > > +{
> > > > > > > > +	struct drm_dp_mst_topology_mgr *mgr;
> > > > > > > > +	struct drm_dp_mst_topology_state *mst_state;
> > > > > > > > +	int i, ret = 0;
> > > > > > > > +
> > > > > > > > +	for_each_new_mst_mgr_in_state(state, mgr, mst_state, i) {
> > > > > > > > +		ret = drm_dp_mst_atomic_check_topology_state(mgr,
> > > > > > > > mst_state);
> > > > > > > > +		if (ret)
> > > > > > > > +			break;
> > > > > > > > +	}
> > > > > > > > +
> > > > > > > > +	return ret;
> > > > > > > > +}
> > > > > > > > +EXPORT_SYMBOL(drm_dp_mst_atomic_check);
> > > > > > > > +
> > > > > > > > +const struct drm_private_state_funcs
> > > > > > > > drm_dp_mst_topology_state_funcs =
> > > > > > > > {
> > > > > > > >  	.atomic_duplicate_state = drm_dp_mst_duplicate_state,
> > > > > > > >  	.atomic_destroy_state = drm_dp_mst_destroy_state,
> > > > > > > >  };
> > > > > > > > +EXPORT_SYMBOL(drm_dp_mst_topology_state_funcs);
> > > > > > > > 
> > > > > > > >  /**
> > > > > > > >   * drm_atomic_get_mst_topology_state: get MST topology state
> > > > > > > > @@ -3216,13 +3444,11 @@ int drm_dp_mst_topology_mgr_init(struct
> > > > > > > > drm_dp_mst_topology_mgr *mgr,
> > > > > > > >  		return -ENOMEM;
> > > > > > > > 
> > > > > > > >  	mst_state->mgr = mgr;
> > > > > > > > -
> > > > > > > > -	/* max. time slots - one slot for MTP header */
> > > > > > > > -	mst_state->avail_slots = 63;
> > > > > > > > +	INIT_LIST_HEAD(&mst_state->vcpis);
> > > > > > > > 
> > > > > > > >  	drm_atomic_private_obj_init(&mgr->base,
> > > > > > > >  				    &mst_state->base,
> > > > > > > > -				    &mst_state_funcs);
> > > > > > > > +				    &drm_dp_mst_topology_state_funcs);
> > > > > > > > 
> > > > > > > >  	return 0;
> > > > > > > >  }
> > > > > > > > diff --git a/drivers/gpu/drm/i915/intel_display.c
> > > > > > > > b/drivers/gpu/drm/i915/intel_display.c
> > > > > > > > index 132e978227fb..8e85e04a5948 100644
> > > > > > > > --- a/drivers/gpu/drm/i915/intel_display.c
> > > > > > > > +++ b/drivers/gpu/drm/i915/intel_display.c
> > > > > > > > @@ -12592,6 +12592,10 @@ static int intel_atomic_check(struct
> > > > > > > > drm_device
> > > > > > > > *dev,
> > > > > > > >  				       "[modeset]" : "[fastset]");
> > > > > > > >  	}
> > > > > > > > 
> > > > > > > > +	ret = drm_dp_mst_atomic_check(state);
> > > > > > > > +	if (ret)
> > > > > > > > +		return ret;
> > > > > > > > +
> > > > > > > >  	if (any_ms) {
> > > > > > > >  		ret = intel_modeset_checks(state);
> > > > > > > > 
> > > > > > > > diff --git a/drivers/gpu/drm/i915/intel_dp_mst.c
> > > > > > > > b/drivers/gpu/drm/i915/intel_dp_mst.c
> > > > > > > > index 4de247ddf05f..be2d1ac7d87d 100644
> > > > > > > > --- a/drivers/gpu/drm/i915/intel_dp_mst.c
> > > > > > > > +++ b/drivers/gpu/drm/i915/intel_dp_mst.c
> > > > > > > > @@ -41,8 +41,13 @@ static bool
> > > > > > > > intel_dp_mst_compute_config(struct
> > > > > > > > intel_encoder *encoder,
> > > > > > > >  	struct drm_connector *connector = conn_state->connector;
> > > > > > > >  	void *port = to_intel_connector(connector)->port;
> > > > > > > >  	struct drm_atomic_state *state = pipe_config->base.state;
> > > > > > > > +	struct drm_crtc *crtc = pipe_config->base.crtc;
> > > > > > > > +	struct drm_crtc_state *old_crtc_state =
> > > > > > > > +		drm_atomic_get_old_crtc_state(state, crtc);
> > > > > > > > +	struct drm_crtc_state *new_crtc_state = &pipe_config->base;
> > > > > > > >  	int bpp;
> > > > > > > > -	int lane_count, slots = 0;
> > > > > > > > +	int lane_count, slots =
> > > > > > > > +		to_intel_crtc_state(old_crtc_state)->dp_m_n.tu;
> > > > > > > >  	const struct drm_display_mode *adjusted_mode = &pipe_config-
> > > > > > > > > base.adjusted_mode;
> > > > > > > >  	int mst_pbn;
> > > > > > > >  	bool constant_n = drm_dp_has_quirk(&intel_dp->desc,
> > > > > > > > @@ -77,8 +82,12 @@ static bool
> > > > > > > > intel_dp_mst_compute_config(struct
> > > > > > > > intel_encoder *encoder,
> > > > > > > >  	mst_pbn = drm_dp_calc_pbn_mode(adjusted_mode->crtc_clock,
> > > > > > > > bpp);
> > > > > > > >  	pipe_config->pbn = mst_pbn;
> > > > > > > > 
> > > > > > > > -	/* Zombie connectors can't have VCPI slots */
> > > > > > > > -	if (!drm_connector_is_unregistered(connector)) {
> > > > > > > > +	/* Only change VCPI allocation on actual mode changes, to
> > > > > > > > prevent us
> > > > > > > > +	 * from trying to allocate VCPI to ports that no longer exist
> > > > > > > > when we
> > > > > > > > +	 * may just be trying to disable DPMS on them
> > > > > > > > +	 */
> > > > > > > > +	if (new_crtc_state->mode_changed ||
> > > > > > > > +	    new_crtc_state->connectors_changed) {
> > > > > > > 
> > > > > > > Why exactly is this needed? We only call compute_config on the
> > > > > > > enocoder
> > > > > > > when enabling an output. Not when disabling one. Similar for the
> > > > > > > atomic
> > > > > > > helpers and the various encoder callbacks (you have the same
> > > > > > > change in
> > > > > > > the
> > > > > > > nouveau code). Did this actually blow up while testing?
> > > > > > > 
> > > > > > > >  		slots = drm_dp_atomic_find_vcpi_slots(state,
> > > > > > > >  						      &intel_dp-
> > > > > > > > > mst_mgr,
> > > > > > > >  						      port,
> > > > > > > > @@ -107,36 +116,39 @@ static bool
> > > > > > > > intel_dp_mst_compute_config(struct
> > > > > > > > intel_encoder *encoder,
> > > > > > > >  	return true;
> > > > > > > >  }
> > > > > > > > 
> > > > > > > > -static int intel_dp_mst_atomic_check(struct drm_connector
> > > > > > > > *connector,
> > > > > > > > -		struct drm_connector_state *new_conn_state)
> > > > > > > > +static int
> > > > > > > > +intel_dp_mst_atomic_check(struct drm_connector *connector,
> > > > > > > > +			  struct drm_connector_state *new_conn_state)
> > > > > > > >  {
> > > > > > > > +	struct intel_connector *intel_connector =
> > > > > > > > +		to_intel_connector(connector);
> > > > > > > > +	struct drm_dp_mst_topology_mgr *mgr =
> > > > > > > > +		&intel_connector->mst_port->mst_mgr;
> > > > > > > > +	struct drm_dp_mst_port *port = intel_connector->port;
> > > > > > > >  	struct drm_atomic_state *state = new_conn_state->state;
> > > > > > > > -	struct drm_connector_state *old_conn_state;
> > > > > > > > -	struct drm_crtc *old_crtc;
> > > > > > > > -	struct drm_crtc_state *crtc_state;
> > > > > > > > -	int slots, ret = 0;
> > > > > > > > +	struct drm_connector_state *old_conn_state =
> > > > > > > > +		drm_atomic_get_old_connector_state(state, connector);
> > > > > > > > +	struct drm_crtc *old_crtc = old_conn_state->crtc,
> > > > > > > > +			*new_crtc = new_conn_state->crtc;
> > > > > > > > +	struct drm_crtc_state *old_crtc_state, *new_crtc_state;
> > > > > > > > 
> > > > > > > > -	old_conn_state = drm_atomic_get_old_connector_state(state,
> > > > > > > > connector);
> > > > > > > > -	old_crtc = old_conn_state->crtc;
> > > > > > > 
> > > > > > > Bikeshed: Reworking all the above hides the real changes quite a
> > > > > > > bit.
> > > > > > > 
> > > > > > > >  	if (!old_crtc)
> > > > > > > > -		return ret;
> > > > > > > > +		return 0;
> > > > > > > > 
> > > > > > > > -	crtc_state = drm_atomic_get_new_crtc_state(state, old_crtc);
> > > > > > > > -	slots = to_intel_crtc_state(crtc_state)->dp_m_n.tu;
> > > > > > > > -	if (drm_atomic_crtc_needs_modeset(crtc_state) && slots > 0) {
> > > > > > > > -		struct drm_dp_mst_topology_mgr *mgr;
> > > > > > > > -		struct drm_encoder *old_encoder;
> > > > > > > > +	old_crtc_state = drm_atomic_get_old_crtc_state(state,
> > > > > > > > old_crtc);
> > > > > > > > +	if (!old_crtc_state ||
> > > > > > > 
> > > > > > > !old_crtc_state would be a framework bug. Just remove that check.
> > > > > > > 
> > > > > > > > +	    !to_intel_crtc_state(old_crtc_state)->dp_m_n.tu)
> > > > > > > > +		return 0;
> > > > > > > 
> > > > > > > Hm, why do we need this? Having a DP port previously enabled with
> > > > > > > a 0
> > > > > > > tu
> > > > > > > sounds like a pretty big driver bug.
> > > > > > > > -		old_encoder = old_conn_state->best_encoder;
> > > > > > > > -		mgr = &enc_to_mst(old_encoder)->primary->dp.mst_mgr;
> > > > > > > > +	/* If we switched CRTCs, clear the previous one's allocation
> > > > > > > > */
> > > > > > > > +	new_crtc_state = drm_atomic_get_new_crtc_state(state,
> > > > > > > > old_crtc);
> > > > > > > > +	if (new_crtc_state->connectors_changed && !new_crtc_state-
> > > > > > > > > enable)
> > > > > > > > +		to_intel_crtc_state(new_crtc_state)->dp_m_n.tu = 0;
> > > > > > > 
> > > > > > > No idea why we need to clear this here? Shouldn't we just clear
> > > > > > > this
> > > > > > > unconditionally if we release the allocation.
> > > > > > > 
> > > > > > > Also naming convention, I'd call this old_crtc_new_state, and
> > > > > > > maybe
> > > > > > > old_crtc_old_state. To make it clear these are the old/new states
> > > > > > > for
> > > > > > > old_crtc. Instead of old/new state for the new crtc.
> > > > > > > 
> > > > > > > > -		ret = drm_dp_atomic_release_vcpi_slots(state, mgr,
> > > > > > > > slots);
> > > > > > > > -		if (ret)
> > > > > > > > -			DRM_DEBUG_KMS("failed releasing %d vcpi
> > > > > > > > slots:%d\n",
> > > > > > > > slots, ret);
> > > > > > > > -		else
> > > > > > > > -			to_intel_crtc_state(crtc_state)->dp_m_n.tu =
> > > > > > > > 0;
> > > > > > > > -	}
> > > > > > > > -	return ret;
> > > > > > > > +	if (new_crtc)
> > > > > > > > +		return 0;
> > > > > > > 
> > > > > > > Not sure why we shouldn't release this? If we handle this in
> > > > > > > helpers,
> > > > > > > it
> > > > > > > would remove piles of special casing code here I think. Plus
> > > > > > > remove
> > > > > > > special code in the hlpers.
> > > > > > > 
> > > > > > > 
> > > > > > > > +
> > > > > > > > +	return drm_dp_atomic_release_vcpi_slots(state, mgr, port);
> > > > > > > >  }
> > > > > > > > 
> > > > > > > >  static void intel_mst_disable_dp(struct intel_encoder *encoder,
> > > > > > > > diff --git a/include/drm/drm_dp_mst_helper.h
> > > > > > > > b/include/drm/drm_dp_mst_helper.h
> > > > > > > > index 3faceb66f5cb..2392e12b6bce 100644
> > > > > > > > --- a/include/drm/drm_dp_mst_helper.h
> > > > > > > > +++ b/include/drm/drm_dp_mst_helper.h
> > > > > > > > @@ -406,9 +406,16 @@ struct drm_dp_payload {
> > > > > > > > 
> > > > > > > >  #define to_dp_mst_topology_state(x) container_of(x, struct
> > > > > > > > drm_dp_mst_topology_state, base)
> > > > > > > > 
> > > > > > > > +struct drm_dp_vcpi_allocation {
> > > > > > > 
> > > > > > > I think a comment here that this pointer holds a full reference
> > > > > > > (even
> > > > > > > past
> > > > > > > when the connector is unplugged) would be good. Otherwise the loop
> > > > > > > in
> > > > > > > release_vcpi would be buggy.
> > > > > > > 
> > > > > > > > +	struct drm_dp_mst_port *port;
> > > > > > > > +	int vcpi;
> > > > > > > > +	struct list_head next;
> > > > > > > > +};
> > > > > > > > +
> > > > > > > >  struct drm_dp_mst_topology_state {
> > > > > > > >  	struct drm_private_state base;
> > > > > > > > -	int avail_slots;
> > > > > > > > +	struct list_head vcpis;
> > > > > > > > +	u8 vcpi_allocated;
> > > > > > > >  	struct drm_dp_mst_topology_mgr *mgr;
> > > > > > > >  };
> > > > > > > > 
> > > > > > > > @@ -619,14 +626,17 @@ void
> > > > > > > > drm_dp_mst_topology_mgr_suspend(struct
> > > > > > > > drm_dp_mst_topology_mgr *mgr);
> > > > > > > >  int drm_dp_mst_topology_mgr_resume(struct
> > > > > > > > drm_dp_mst_topology_mgr
> > > > > > > > *mgr);
> > > > > > > >  struct drm_dp_mst_topology_state
> > > > > > > > *drm_atomic_get_mst_topology_state(struct drm_atomic_state
> > > > > > > > *state,
> > > > > > > >  								    st
> > > > > > > > ruct
> > > > > > > > drm_dp_mst_topology_mgr *mgr);
> > > > > > > > -int drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state
> > > > > > > > *state,
> > > > > > > > -				  struct drm_dp_mst_topology_mgr *mgr,
> > > > > > > > -				  struct drm_dp_mst_port *port, int
> > > > > > > > pbn);
> > > > > > > > -int drm_dp_atomic_release_vcpi_slots(struct drm_atomic_state
> > > > > > > > *state,
> > > > > > > > -				     struct drm_dp_mst_topology_mgr
> > > > > > > > *mgr,
> > > > > > > > -				     int slots);
> > > > > > > > +int __must_check
> > > > > > > > +drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state *state,
> > > > > > > > +			      struct drm_dp_mst_topology_mgr *mgr,
> > > > > > > > +			      struct drm_dp_mst_port *port, int pbn);
> > > > > > > > +int __must_check
> > > > > > > > +drm_dp_atomic_release_vcpi_slots(struct drm_atomic_state
> > > > > > > > *state,
> > > > > > > > +				 struct drm_dp_mst_topology_mgr *mgr,
> > > > > > > > +				 struct drm_dp_mst_port *port);
> > > > > > > >  int drm_dp_send_power_updown_phy(struct drm_dp_mst_topology_mgr
> > > > > > > > *mgr,
> > > > > > > >  				 struct drm_dp_mst_port *port, bool
> > > > > > > > power_up);
> > > > > > > > +int __must_check drm_dp_mst_atomic_check(struct
> > > > > > > > drm_atomic_state
> > > > > > > > *state);
> > > > > > > > 
> > > > > > > >  extern const struct drm_private_state_funcs
> > > > > > > > drm_dp_mst_topology_state_funcs;
> > > > > > > > 
> > > > > > > > --
> > > > > > > > 2.19.1
> > > > > > > > 
> > > > > > > 
> > > > > > > --
> > > > > > > Daniel Vetter
> > > > > > > Software Engineer, Intel Corporation
> > > > > > > http://blog.ffwll.ch
> > > > > --
> > > > > Cheers,
> > > > > 	Lyude Paul
> > > > > 
> > > --
> > > Cheers,
> > > 	Lyude Paul
> > > 
> -- 
> Cheers,
> 	Lyude Paul
>

Patch

diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
index 00fbe7a2699d..183d0e832ced 100644
--- a/drivers/gpu/drm/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/drm_dp_mst_topology.c
@@ -2617,21 +2617,42 @@  static int drm_dp_init_vcpi(struct drm_dp_mst_topology_mgr *mgr,
 }
 
 /**
- * drm_dp_atomic_find_vcpi_slots() - Find and add vcpi slots to the state
+ * drm_dp_atomic_find_vcpi_slots() - Find and add VCPI slots to the state
  * @state: global atomic state
  * @mgr: MST topology manager for the port
  * @port: port to find vcpi slots for
  * @pbn: bandwidth required for the mode in PBN
  *
- * RETURNS:
- * Total slots in the atomic state assigned for this port or error
+ * Allocates VCPI slots to @port, replacing any previous VCPI allocations it
+ * may have had. Any atomic drivers which support MST must call this function
+ * in their &drm_encoder_helper_funcs.atomic_check() callback to change the
+ * current VCPI allocation for the new state, but only when
+ * &drm_crtc_state.mode_changed or &drm_crtc_state.connectors_changed is set
+ * to ensure compatibility with userspace applications that still use the
+ * legacy modesetting UAPI.
+ *
+ * Allocations set by this function are not checked against the bandwidth
+ * restraints of @mgr until the driver calls drm_dp_mst_atomic_check().
+ *
+ * Additionally, it is OK to call this function multiple times on the same
+ * @port as needed. It is not OK however, to call this function and
+ * drm_dp_atomic_release_vcpi_slots() in the same atomic check phase.
+ *
+ * See also:
+ * drm_dp_atomic_release_vcpi_slots()
+ * drm_dp_mst_atomic_check()
+ *
+ * Returns:
+ * Total slots in the atomic state assigned for this port, or a negative error
+ * code if the port no longer exists
  */
 int drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state *state,
 				  struct drm_dp_mst_topology_mgr *mgr,
 				  struct drm_dp_mst_port *port, int pbn)
 {
 	struct drm_dp_mst_topology_state *topology_state;
-	int req_slots;
+	struct drm_dp_vcpi_allocation *pos, *vcpi = NULL;
+	int prev_slots, req_slots, ret;
 
 	topology_state = drm_atomic_get_mst_topology_state(state, mgr);
 	if (IS_ERR(topology_state))
@@ -2640,20 +2661,60 @@  int drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state *state,
 	port = drm_dp_get_validated_port_ref(mgr, port);
 	if (port == NULL)
 		return -EINVAL;
-	req_slots = DIV_ROUND_UP(pbn, mgr->pbn_div);
-	DRM_DEBUG_KMS("vcpi slots req=%d, avail=%d\n",
-			req_slots, topology_state->avail_slots);
 
-	if (req_slots > topology_state->avail_slots) {
-		drm_dp_put_port(port);
-		return -ENOSPC;
+	topology_state->vcpi_allocated = true;
+
+	/* Find the current allocation for this port, if any */
+	list_for_each_entry(pos, &topology_state->vcpis, next) {
+		if (pos->port == port) {
+			vcpi = pos;
+			prev_slots = vcpi->vcpi;
+
+			/*
+			 * This should never happen, unless the driver tries
+			 * releasing and allocating the same VCPI allocation,
+			 * which is an error
+			 */
+			if (WARN_ON(!prev_slots)) {
+				DRM_ERROR("cannot allocate and release VCPI on [MST PORT:%p] in the same state\n",
+					  port);
+				return -EINVAL;
+			}
+
+			break;
+		}
 	}
+	if (!vcpi)
+		prev_slots = 0;
 
-	topology_state->avail_slots -= req_slots;
-	DRM_DEBUG_KMS("vcpi slots avail=%d", topology_state->avail_slots);
+	req_slots = DIV_ROUND_UP(pbn, mgr->pbn_div);
+
+	DRM_DEBUG_ATOMIC("[CONNECTOR:%d:%s] [MST PORT:%p] VCPI %d -> %d\n",
+			 port->connector->base.id, port->connector->name,
+			 port, prev_slots, req_slots);
+
+	/* Add the new allocation to the state */
+	if (!vcpi) {
+		vcpi = kzalloc(sizeof(*vcpi), GFP_KERNEL);
+		if (!vcpi) {
+			ret = -ENOMEM;
+			goto out;
+		}
 
+		vcpi->port = drm_dp_get_validated_port_ref(mgr, port);
+		if (!vcpi->port) {
+			ret = -EINVAL;
+			kfree(vcpi);
+			goto out;
+		}
+		list_add(&vcpi->next, &topology_state->vcpis);
+	}
+	vcpi->vcpi = req_slots;
+
+	ret = req_slots;
+out:
 	drm_dp_put_port(port);
-	return req_slots;
+	return ret;
 }
 EXPORT_SYMBOL(drm_dp_atomic_find_vcpi_slots);
 
@@ -2661,31 +2722,66 @@  EXPORT_SYMBOL(drm_dp_atomic_find_vcpi_slots);
  * drm_dp_atomic_release_vcpi_slots() - Release allocated vcpi slots
  * @state: global atomic state
  * @mgr: MST topology manager for the port
- * @slots: number of vcpi slots to release
+ * @port: The port to release the VCPI slots from
  *
- * RETURNS:
- * 0 if @slots were added back to &drm_dp_mst_topology_state->avail_slots or
- * negative error code
+ * Releases any VCPI slots that have been allocated to a port in the atomic
+ * state. Any atomic drivers which support MST must call this function in
+ * their &drm_connector_helper_funcs.atomic_check() callback when the
+ * connector will no longer have VCPI allocated (e.g. because it's CRTC was
+ * removed) when it had VCPI allocated in the previous atomic state.
+ *
+ * It is OK to call this even if @port has been removed from the system.
+ * Additionally, it is OK to call this function multiple times on the same
+ * @port as needed. It is not OK however, to call this function and
+ * drm_dp_atomic_find_vcpi_slots() on the same @port in a single atomic check
+ * phase.
+ *
+ * See also:
+ * drm_dp_atomic_find_vcpi_slots()
+ * drm_dp_mst_atomic_check()
+ *
+ * Returns:
+ * 0 if all slots for this port were added back to
+ * &drm_dp_mst_topology_state.avail_slots or negative error code
  */
 int drm_dp_atomic_release_vcpi_slots(struct drm_atomic_state *state,
 				     struct drm_dp_mst_topology_mgr *mgr,
-				     int slots)
+				     struct drm_dp_mst_port *port)
 {
 	struct drm_dp_mst_topology_state *topology_state;
+	struct drm_dp_vcpi_allocation *pos;
+	bool found = false;
 
 	topology_state = drm_atomic_get_mst_topology_state(state, mgr);
 	if (IS_ERR(topology_state))
 		return PTR_ERR(topology_state);
 
-	/* We cannot rely on port->vcpi.num_slots to update
-	 * topology_state->avail_slots as the port may not exist if the parent
-	 * branch device was unplugged. This should be fixed by tracking
-	 * per-port slot allocation in drm_dp_mst_topology_state instead of
-	 * depending on the caller to tell us how many slots to release.
-	 */
-	topology_state->avail_slots += slots;
-	DRM_DEBUG_KMS("vcpi slots released=%d, avail=%d\n",
-			slots, topology_state->avail_slots);
+	list_for_each_entry(pos, &topology_state->vcpis, next) {
+		if (pos->port == port) {
+			found = true;
+			break;
+		}
+	}
+	if (WARN_ON(!found)) {
+		DRM_ERROR("no VCPI for [MST PORT:%p] found in mst state %p\n",
+			  port, &topology_state->base);
+		return -EINVAL;
+	}
+
+	DRM_DEBUG_ATOMIC("[MST PORT:%p] VCPI %d -> 0\n", port, pos->vcpi);
+	if (pos->vcpi) {
+		/*
+		 * This should be safe-as the previous state will still hold a
+		 * reference to this port until we swap states. If it's not,
+		 * the driver has a bookkeeping error
+		 */
+		if (WARN_ON(drm_dp_put_port(port))) {
+			DRM_ERROR("cannot allocate and release VCPI on [MST PORT:%p] in the same state\n",
+				  port);
+			return -EINVAL;
+		}
+		pos->vcpi = 0;
+	}
 
 	return 0;
 }
@@ -3115,15 +3211,45 @@  static void drm_dp_destroy_connector_work(struct work_struct *work)
 static struct drm_private_state *
 drm_dp_mst_duplicate_state(struct drm_private_obj *obj)
 {
-	struct drm_dp_mst_topology_state *state;
+	struct drm_dp_mst_topology_state *state, *old_state =
+		to_dp_mst_topology_state(obj->state);
+	struct drm_dp_vcpi_allocation *pos, *vcpi;
 
-	state = kmemdup(obj->state, sizeof(*state), GFP_KERNEL);
+	state = kmemdup(old_state, sizeof(*state), GFP_KERNEL);
 	if (!state)
 		return NULL;
 
 	__drm_atomic_helper_private_obj_duplicate_state(obj, &state->base);
 
+	INIT_LIST_HEAD(&state->vcpis);
+	state->vcpi_allocated = false;
+
+	list_for_each_entry(pos, &old_state->vcpis, next) {
+		/* Prune leftover freed VCPI allocations */
+		if (!pos->vcpi)
+			continue;
+
+		vcpi = kmemdup(pos, sizeof(*vcpi), GFP_KERNEL);
+		if (!vcpi)
+			goto fail;
+
+		/* Take a plain old kref here and don't validate the port, as
+		 * it's topology might not even exist anymore
+		 */
+		kref_get(&vcpi->port->kref);
+		list_add(&vcpi->next, &state->vcpis);
+	}
+
 	return &state->base;
+
+fail:
+	list_for_each_entry_safe(pos, vcpi, &state->vcpis, next) {
+		drm_dp_put_port(pos->port);
+		kfree(pos);
+	}
+	kfree(state);
+
+	return NULL;
 }
 
 static void drm_dp_mst_destroy_state(struct drm_private_obj *obj,
@@ -3131,14 +3257,116 @@  static void drm_dp_mst_destroy_state(struct drm_private_obj *obj,
 {
 	struct drm_dp_mst_topology_state *mst_state =
 		to_dp_mst_topology_state(state);
+	struct drm_dp_vcpi_allocation *pos, *tmp;
+
+	list_for_each_entry_safe(pos, tmp, &mst_state->vcpis, next) {
+		/* We only keep references to ports with non-zero VCPIs */
+		if (pos->vcpi)
+			drm_dp_put_port(pos->port);
+		kfree(pos);
+	}
 
 	kfree(mst_state);
 }
 
-static const struct drm_private_state_funcs mst_state_funcs = {
+static inline int
+drm_dp_mst_atomic_check_topology_state(struct drm_dp_mst_topology_mgr *mgr,
+				       struct drm_dp_mst_topology_state *mst_state)
+{
+	struct drm_dp_vcpi_allocation *vcpi;
+	struct drm_dp_mst_port *port;
+	int avail_slots = 63, ret;
+
+	/* There's no possible scenario where releasing VCPI or keeping it the
+	 * same would make the state invalid
+	 */
+	if (!mst_state->vcpi_allocated) {
+		DRM_DEBUG_ATOMIC("[MST MGR:%p] state %p allocates no VCPI, check skipped\n",
+				 mgr, &mst_state->base);
+		return 0;
+	}
+
+	list_for_each_entry(vcpi, &mst_state->vcpis, next) {
+		/* Releasing VCPI is always OK-even if the port is gone */
+		if (!vcpi->vcpi) {
+			DRM_DEBUG_ATOMIC("[MST PORT:%p] releases all VCPI slots\n",
+					 vcpi->port);
+			continue;
+		}
+
+		port = drm_dp_get_validated_port_ref(mgr, vcpi->port);
+		if (!port) {
+			DRM_DEBUG_ATOMIC("[MST PORT:%p] is gone but still has VCPI, cannot add new VCPI\n",
+					 vcpi->port);
+			return -EINVAL;
+		}
+
+		DRM_DEBUG_ATOMIC("[MST PORT:%p] requires %d vcpi slots\n",
+				 vcpi->port, vcpi->vcpi);
+
+		avail_slots -= vcpi->vcpi;
+		if (avail_slots < 0) {
+			DRM_DEBUG_ATOMIC("[MST PORT:%p] not enough VCPI slots in mst state %p (avail=%d)\n",
+					 vcpi->port, mst_state,
+					 avail_slots + vcpi->vcpi);
+			ret = -ENOSPC;
+			goto port_fail;
+		}
+
+		drm_dp_put_port(port);
+	}
+	DRM_DEBUG_ATOMIC("[MST MGR:%p] mst state %p VCPI avail=%d used=%d\n",
+			 mgr, mst_state, avail_slots,
+			 63 - avail_slots);
+
+	return 0;
+port_fail:
+	drm_dp_put_port(port);
+	return ret;
+}
+
+/**
+ * drm_dp_mst_atomic_check - Check that the new state of an MST topology in an
+ * atomic update is valid
+ * @state: Pointer to the new &struct drm_dp_mst_topology_state
+ *
+ * Checks the given topology state for an atomic update to ensure that it's
+ * valid. This includes checking whether there's enough bandwidth to support
+ * the new VCPI allocations in the atomic update.
+ *
+ * Any atomic drivers supporting DP MST must make sure to call this after
+ * checking the rest of their state in their
+ * &drm_mode_config_funcs.atomic_check() callback.
+ *
+ * See also:
+ * drm_dp_atomic_find_vcpi_slots()
+ * drm_dp_atomic_release_vcpi_slots()
+ *
+ * Returns:
+ *
+ * 0 if the new state is valid, negative error code otherwise.
+ */
+int drm_dp_mst_atomic_check(struct drm_atomic_state *state)
+{
+	struct drm_dp_mst_topology_mgr *mgr;
+	struct drm_dp_mst_topology_state *mst_state;
+	int i, ret = 0;
+
+	for_each_new_mst_mgr_in_state(state, mgr, mst_state, i) {
+		ret = drm_dp_mst_atomic_check_topology_state(mgr, mst_state);
+		if (ret)
+			break;
+	}
+
+	return ret;
+}
+EXPORT_SYMBOL(drm_dp_mst_atomic_check);
+
+const struct drm_private_state_funcs drm_dp_mst_topology_state_funcs = {
 	.atomic_duplicate_state = drm_dp_mst_duplicate_state,
 	.atomic_destroy_state = drm_dp_mst_destroy_state,
 };
+EXPORT_SYMBOL(drm_dp_mst_topology_state_funcs);
 
 /**
  * drm_atomic_get_mst_topology_state: get MST topology state
@@ -3216,13 +3444,11 @@  int drm_dp_mst_topology_mgr_init(struct drm_dp_mst_topology_mgr *mgr,
 		return -ENOMEM;
 
 	mst_state->mgr = mgr;
-
-	/* max. time slots - one slot for MTP header */
-	mst_state->avail_slots = 63;
+	INIT_LIST_HEAD(&mst_state->vcpis);
 
 	drm_atomic_private_obj_init(&mgr->base,
 				    &mst_state->base,
-				    &mst_state_funcs);
+				    &drm_dp_mst_topology_state_funcs);
 
 	return 0;
 }
diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index 132e978227fb..8e85e04a5948 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -12592,6 +12592,10 @@  static int intel_atomic_check(struct drm_device *dev,
 				       "[modeset]" : "[fastset]");
 	}
 
+	ret = drm_dp_mst_atomic_check(state);
+	if (ret)
+		return ret;
+
 	if (any_ms) {
 		ret = intel_modeset_checks(state);
 
diff --git a/drivers/gpu/drm/i915/intel_dp_mst.c b/drivers/gpu/drm/i915/intel_dp_mst.c
index 4de247ddf05f..be2d1ac7d87d 100644
--- a/drivers/gpu/drm/i915/intel_dp_mst.c
+++ b/drivers/gpu/drm/i915/intel_dp_mst.c
@@ -41,8 +41,13 @@  static bool intel_dp_mst_compute_config(struct intel_encoder *encoder,
 	struct drm_connector *connector = conn_state->connector;
 	void *port = to_intel_connector(connector)->port;
 	struct drm_atomic_state *state = pipe_config->base.state;
+	struct drm_crtc *crtc = pipe_config->base.crtc;
+	struct drm_crtc_state *old_crtc_state =
+		drm_atomic_get_old_crtc_state(state, crtc);
+	struct drm_crtc_state *new_crtc_state = &pipe_config->base;
 	int bpp;
-	int lane_count, slots = 0;
+	int lane_count, slots =
+		to_intel_crtc_state(old_crtc_state)->dp_m_n.tu;
 	const struct drm_display_mode *adjusted_mode = &pipe_config->base.adjusted_mode;
 	int mst_pbn;
 	bool constant_n = drm_dp_has_quirk(&intel_dp->desc,
@@ -77,8 +82,12 @@  static bool intel_dp_mst_compute_config(struct intel_encoder *encoder,
 	mst_pbn = drm_dp_calc_pbn_mode(adjusted_mode->crtc_clock, bpp);
 	pipe_config->pbn = mst_pbn;
 
-	/* Zombie connectors can't have VCPI slots */
-	if (!drm_connector_is_unregistered(connector)) {
+	/* Only change VCPI allocation on actual mode changes, to prevent us
+	 * from trying to allocate VCPI to ports that no longer exist when we
+	 * may just be trying to disable DPMS on them
+	 */
+	if (new_crtc_state->mode_changed ||
+	    new_crtc_state->connectors_changed) {
 		slots = drm_dp_atomic_find_vcpi_slots(state,
 						      &intel_dp->mst_mgr,
 						      port,
@@ -107,36 +116,39 @@  static bool intel_dp_mst_compute_config(struct intel_encoder *encoder,
 	return true;
 }
 
-static int intel_dp_mst_atomic_check(struct drm_connector *connector,
-		struct drm_connector_state *new_conn_state)
+static int
+intel_dp_mst_atomic_check(struct drm_connector *connector,
+			  struct drm_connector_state *new_conn_state)
 {
+	struct intel_connector *intel_connector =
+		to_intel_connector(connector);
+	struct drm_dp_mst_topology_mgr *mgr =
+		&intel_connector->mst_port->mst_mgr;
+	struct drm_dp_mst_port *port = intel_connector->port;
 	struct drm_atomic_state *state = new_conn_state->state;
-	struct drm_connector_state *old_conn_state;
-	struct drm_crtc *old_crtc;
-	struct drm_crtc_state *crtc_state;
-	int slots, ret = 0;
+	struct drm_connector_state *old_conn_state =
+		drm_atomic_get_old_connector_state(state, connector);
+	struct drm_crtc *old_crtc = old_conn_state->crtc,
+			*new_crtc = new_conn_state->crtc;
+	struct drm_crtc_state *old_crtc_state, *new_crtc_state;
 
-	old_conn_state = drm_atomic_get_old_connector_state(state, connector);
-	old_crtc = old_conn_state->crtc;
 	if (!old_crtc)
-		return ret;
+		return 0;
 
-	crtc_state = drm_atomic_get_new_crtc_state(state, old_crtc);
-	slots = to_intel_crtc_state(crtc_state)->dp_m_n.tu;
-	if (drm_atomic_crtc_needs_modeset(crtc_state) && slots > 0) {
-		struct drm_dp_mst_topology_mgr *mgr;
-		struct drm_encoder *old_encoder;
+	old_crtc_state = drm_atomic_get_old_crtc_state(state, old_crtc);
+	if (!old_crtc_state ||
+	    !to_intel_crtc_state(old_crtc_state)->dp_m_n.tu)
+		return 0;
 
-		old_encoder = old_conn_state->best_encoder;
-		mgr = &enc_to_mst(old_encoder)->primary->dp.mst_mgr;
+	/* If we switched CRTCs, clear the previous one's allocation */
+	new_crtc_state = drm_atomic_get_new_crtc_state(state, old_crtc);
+	if (new_crtc_state->connectors_changed && !new_crtc_state->enable)
+		to_intel_crtc_state(new_crtc_state)->dp_m_n.tu = 0;
 
-		ret = drm_dp_atomic_release_vcpi_slots(state, mgr, slots);
-		if (ret)
-			DRM_DEBUG_KMS("failed releasing %d vcpi slots:%d\n", slots, ret);
-		else
-			to_intel_crtc_state(crtc_state)->dp_m_n.tu = 0;
-	}
-	return ret;
+	if (new_crtc)
+		return 0;
+
+	return drm_dp_atomic_release_vcpi_slots(state, mgr, port);
 }
 
 static void intel_mst_disable_dp(struct intel_encoder *encoder,
diff --git a/include/drm/drm_dp_mst_helper.h b/include/drm/drm_dp_mst_helper.h
index 3faceb66f5cb..2392e12b6bce 100644
--- a/include/drm/drm_dp_mst_helper.h
+++ b/include/drm/drm_dp_mst_helper.h
@@ -406,9 +406,16 @@  struct drm_dp_payload {
 
 #define to_dp_mst_topology_state(x) container_of(x, struct drm_dp_mst_topology_state, base)
 
+struct drm_dp_vcpi_allocation {
+	struct drm_dp_mst_port *port;
+	int vcpi;
+	struct list_head next;
+};
+
 struct drm_dp_mst_topology_state {
 	struct drm_private_state base;
-	int avail_slots;
+	struct list_head vcpis;
+	u8 vcpi_allocated;
 	struct drm_dp_mst_topology_mgr *mgr;
 };
 
@@ -619,14 +626,17 @@  void drm_dp_mst_topology_mgr_suspend(struct drm_dp_mst_topology_mgr *mgr);
 int drm_dp_mst_topology_mgr_resume(struct drm_dp_mst_topology_mgr *mgr);
 struct drm_dp_mst_topology_state *drm_atomic_get_mst_topology_state(struct drm_atomic_state *state,
 								    struct drm_dp_mst_topology_mgr *mgr);
-int drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state *state,
-				  struct drm_dp_mst_topology_mgr *mgr,
-				  struct drm_dp_mst_port *port, int pbn);
-int drm_dp_atomic_release_vcpi_slots(struct drm_atomic_state *state,
-				     struct drm_dp_mst_topology_mgr *mgr,
-				     int slots);
+int __must_check
+drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state *state,
+			      struct drm_dp_mst_topology_mgr *mgr,
+			      struct drm_dp_mst_port *port, int pbn);
+int __must_check
+drm_dp_atomic_release_vcpi_slots(struct drm_atomic_state *state,
+				 struct drm_dp_mst_topology_mgr *mgr,
+				 struct drm_dp_mst_port *port);
 int drm_dp_send_power_updown_phy(struct drm_dp_mst_topology_mgr *mgr,
 				 struct drm_dp_mst_port *port, bool power_up);
+int __must_check drm_dp_mst_atomic_check(struct drm_atomic_state *state);
 
 extern const struct drm_private_state_funcs drm_dp_mst_topology_state_funcs;